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Heavier GF16 having trouble getting on plane

Posted: Tue Sep 06, 2022 2:19 pm
by AndrewS
Hello, I finished building my GF16 last year and have taken it out a few times since then but have only managed to get on plane once when we were on a river and it was near glass flat. I used 1708 and and 2 extra gallons of epoxy or so when building the boat so I think it is probably heavier than the original design. I tried adding a doelfin but that did not help nor did an adjustable jack plate. I have been reading through some posts about the gf16 and have seen where people have swapped the prop and had better luck getting on plane. The boat has a 20hp merc 4 stroke efi motor on it and I am not 100% sure if something like this 10 pitch prop is what I should try out : https://www.nettleprops.com/store/p/876 ... rcury.aspx

Any help would be appreciated.

Re: Heavier GF16 having trouble getting on plane

Posted: Tue Sep 06, 2022 2:29 pm
by Jaysen
There is a good prop guy here... Tomw1. He can get you the spec on the right prop, but he will need some info.

What's the total weight of the boat AS YOU BUILT IT including all the cargo you would carry?

1. If you have a couple medium guys, food, drink, fuel, ice, bait etc. add that up.
2. Take the boat on a trailer to a scale and get it weighed. Then take the empty trailer to the scale and weigh it (if you have an unmodified trailer look for the weight on the manufacturer's label to avoid a second scale).
3. Add those two numbers. This is your total weight.

As much as we think we can use material used, it really does need to be weighed on a scale.

Re: Heavier GF16 having trouble getting on plane

Posted: Wed Sep 07, 2022 1:06 am
by Fuzz
20 hp should be plenty to get that boat on step. Plans only call for 15 hp. Post a picture of how the motor is mounted. I am thinking it is mounted way too low.

Re: Heavier GF16 having trouble getting on plane

Posted: Wed Sep 07, 2022 6:30 am
by cape man
Actually you subtract the trailer weight, not add it. :lol: Jeff does the books and the pricing I hope! 8) :D

What prop are you currently running? Pics of the motor cavitation plate and the bottom of the stern would be good too. Agree with Fuzz. That motor should be plenty.

Re: Heavier GF16 having trouble getting on plane

Posted: Wed Sep 07, 2022 10:19 am
by fallguy1000
AndrewS wrote: Tue Sep 06, 2022 2:19 pm Hello, I finished building my GF16 last year and have taken it out a few times since then but have only managed to get on plane once when we were on a river and it was near glass flat. I used 1708 and and 2 extra gallons of epoxy or so when building the boat so I think it is probably heavier than the original design. I tried adding a doelfin but that did not help nor did an adjustable jack plate. I have been reading through some posts about the gf16 and have seen where people have swapped the prop and had better luck getting on plane. The boat has a 20hp merc 4 stroke efi motor on it and I am not 100% sure if something like this 10 pitch prop is what I should try out : https://www.nettleprops.com/store/p/876 ... rcury.aspx

Any help would be appreciated.
You really need to know if the motor is achieving max rpms. If the prop pitch is wrong; the engine nevers revs up. But TomW1 knows the configuration well enough to spec it close.

What prop is on it now?

Another issue could be if you built in some hook...this is easy to check for as well.

Re: Heavier GF16 having trouble getting on plane

Posted: Wed Sep 07, 2022 12:03 pm
by TomW1
Tom here I will be glad to help you. As Jaysen said I will need the total weight of the boat as it leaves the dock.

Weight of boat:
Motor Merc 20HP 4st I will get the specs I need from there web site
Gas tank gal x 6.3lbs
People weight =
Gear: weight coolers. Ice, fishing gear, and any other things you carry onto the boat
Safety gear 50lbs
Other: Anchors, ropes spare parts like a prop and toolbox.

Be as close as you can with the weights as they are the most important factor in my calculations.

I have been on a GF16 and know you should not be having the problems you are. What size prop do now have? With a 20HP you should have a top speed in the mii 20's or slightly higher. On the GF16 I was on it had a 25HP and we reached the low 30's with two big guys on it. Make sure you have the cavitation plate is even with the bottom of the boat as the others have said.

I am getting ready to go on vacation so it may be next week till I get back to Io you.

Tom

Re: Heavier GF16 having trouble getting on plane

Posted: Wed Sep 07, 2022 7:17 pm
by AndrewS
Thanks for all the replied. I won't be able to weigh it until this weekend. The prop that is on there is a 3 blade and it stamped 10p. I made sure there wasn't a hook on the bottom before we flipped it. Here are some of the photos I was able to take. The top of the cav plate seems dead in line with the bottom of the boat. https://photos.app.goo.gl/MTBUFpYKkaqhaSJm9

Re: Heavier GF16 having trouble getting on plane

Posted: Wed Sep 07, 2022 7:30 pm
by Evan_Gatehouse
Can we assume the prop hub isn't slipping?

Re: Heavier GF16 having trouble getting on plane

Posted: Wed Sep 07, 2022 8:29 pm
by AndrewS
I don't think so, it seems like its pushing water fine, how would I check exactly?

Re: Heavier GF16 having trouble getting on plane

Posted: Thu Sep 08, 2022 8:16 am
by joe2700
AndrewS wrote: Wed Sep 07, 2022 8:29 pm I don't think so, it seems like its pushing water fine, how would I check exactly?
One option is to put a mark on the prop and the hub, use the boat for a while, then see if they still line up.

Re: Heavier GF16 having trouble getting on plane

Posted: Thu Sep 08, 2022 8:48 am
by fallguy1000
I had a 14' Lund aluminum that would not plane with two big guys and some gear with 15hp.

Do the hub mark check.

Check the performance with one person.

Run the jack plate all the way down.

See what happens with a pitch change.

Re: Heavier GF16 having trouble getting on plane

Posted: Thu Sep 08, 2022 9:47 am
by bklake
Going by one of your picture, you have a lot of weight on the transom. Engine, bracket, trolling motor. I would move stuff around to balance the boat fore and aft, moving weight forward. I'm guessing it just plows and cannot get over the hump.

I have a Carolina Skiff J14 and it is frustratingly difficult to get it balanced so it will plane. Based on posts on their old forum, almost everyone with the J14 boat had problems getting it to plane. I finally fixed it the old fashion way by putting a 25hp engine on it but it was scary to go more than 3/4 throttle. I'm back to a 9.9 and have to shift things around to rebalance so it won't plow.

If you don't know your RPM, you will never get the prop dialed in. I found a cheap electronic tach on amazon. Tiny-Tach is the well known brand but there are many knock offs now. It has a signal wire that wraps around one of the spark plug wires. It is very simple to install, the hardest part is working your fingers in the crowded space of the engine. 10 pitch seems a little too steep. I know the Suzuki 20 comes with a 9" pitch 3 blade prop. You must know what your RPM is a full throttle. Did I mention RPM?

Re: Heavier GF16 having trouble getting on plane

Posted: Thu Sep 08, 2022 10:04 am
by fallguy1000
bklake wrote: Thu Sep 08, 2022 9:47 am Going by one of your picture, you have a lot of weight on the transom. Engine, bracket, trolling motor. I would move stuff around to balance the boat fore and aft, moving weight forward. I'm guessing it just plows and cannot get over the hump.

I have a Carolina Skiff J14 and it is frustratingly difficult to get it balanced so it will plane. Based on posts on their old forum, almost everyone with the J14 boat had problems getting it to plane. I finally fixed it the old fashion way by putting a 25hp engine on it but it was scary to go more than 3/4 throttle. I'm back to a 9.9 and have to shift things around to rebalance so it won't plow.

If you don't know your RPM, you will never get the prop dialed in. I found a cheap electronic tach on amazon. Tiny-Tach is the well known brand but there are many knock offs now. It has a signal wire that wraps around one of the spark plug wires. It is very simple to install, the hardest part is working your fingers in the crowded space of the engine. 10 pitch seems a little too steep. I know the Suzuki 20 comes with a 9" pitch 3 blade prop. You must know what your RPM is a full throttle. Did I mention RPM?
Great post! If he can find out wot rpm, he can get the best prop..

Re: Heavier GF16 having trouble getting on plane

Posted: Thu Sep 08, 2022 2:43 pm
by Fuzz
I picked up a Tiny Tac just last week off Ebay. Good price and the seller was great to deal with.
https://www.ebay.com/itm/255413525300

Re: Heavier GF16 having trouble getting on plane

Posted: Thu Sep 08, 2022 5:14 pm
by TomW1
blake I do not see a trolling motor on his boat. Could you confirm this AndrewS? Tom

Re: Heavier GF16 having trouble getting on plane

Posted: Thu Sep 08, 2022 5:18 pm
by TomW1
Fuzz wrote: Thu Sep 08, 2022 2:43 pm I picked up a Tiny Tac just last week off Ebay. Good price and the seller was great to deal with.
https://www.ebay.com/itm/255413525300
Yes, you need some type of tach to dial in the rpm's and the motor to the prop if you do not have one get one. Tom

Re: Heavier GF16 having trouble getting on plane

Posted: Thu Sep 08, 2022 9:21 pm
by bklake
TomW1 wrote: Thu Sep 08, 2022 5:14 pm blake I do not see a trolling motor on his boat. Could you confirm this AndrewS? Tom
Post number 5 has a link to pictures. I was going by that.

Re: Heavier GF16 having trouble getting on plane

Posted: Thu Sep 08, 2022 9:43 pm
by TomW1
bklake wrote: Thu Sep 08, 2022 9:21 pm
TomW1 wrote: Thu Sep 08, 2022 5:14 pm blake I do not see a trolling motor on his boat. Could you confirm this AndrewS? Tom
Post number 5 has a link to pictures. I was going by that.
See it now. Thanks Tom. It is going to complicate my weight calculations and make his balancing the boat weight harder. He has a lot of weight on the transom depending on where the batteries are for both motors. Tom

Re: Heavier GF16 having trouble getting on plane

Posted: Thu Sep 08, 2022 10:02 pm
by bklake
Fuzz wrote: Thu Sep 08, 2022 2:43 pm I picked up a Tiny Tac just last week off Ebay. Good price and the seller was great to deal with.
https://www.ebay.com/itm/255413525300
I got one on amazon that has a battery. It only has one thin wire. I used a Tiny-Tach before and the extra wires for power were a pain to get under/through the cowl. The one I got was supposed to be water resistant. It probably was for the first 5 minutes of its life but then the seals dissolved in water and it leaked :lol: I remember the Tiny Tach being well potted and it didn't leak for a long time.

Re: Heavier GF16 having trouble getting on plane

Posted: Thu Sep 08, 2022 10:52 pm
by AndrewS
I ordered a tinytach on amazon, thanks for the recommendation. as far as weight distribution, I have the 2 6 gal takes up front, I have 2 house batteries in the middle and 1 trolling motor battery under the rear seat.

Re: Heavier GF16 having trouble getting on plane

Posted: Thu Sep 08, 2022 11:07 pm
by TomW1
Andrew I am starting to see a lot of problems for why you cannot get up on plane with your GF16. First is all the weight on your transom with the trolling motor and I assume its battery near it. Mount it on the bow or get rid of it for now. Your Merc can troll at 2-4 mph. Get a tach so you can tell that you are down at low enough rpm's. You need to balance the weight on the stern and the weight on the bow. On the GF16 I was on the Battery and gas tank were under the center seat. The easiest way to do this is add all the weight you have on the stern and how much you have in the bow. Then try to distribute it. I do not know where the Longitudinal Center of Gravity is on the GF16 but probably about the center seat so all weight should be distributed ahead of and behind that point. Tom

Re: Heavier GF16 having trouble getting on plane

Posted: Sat Sep 17, 2022 12:03 pm
by AndrewS
So the boat + trailer weighs 1060lbs, the trailer isn't listed on academy's website anymore and I think it weighs around 220 lbs. Weather permitting I will be taking it out tomorrow morning to get the RPM reading.

Re: Heavier GF16 having trouble getting on plane

Posted: Sun Sep 18, 2022 11:53 am
by TomW1
Andrew there should be a tag on trailer, metal or paper giving the weight of the trailer and tare weight. Tom

Re: Heavier GF16 having trouble getting on plane

Posted: Tue Sep 20, 2022 9:25 pm
by AndrewS
TomW1 wrote: Sun Sep 18, 2022 11:53 am Andrew there should be a tag on trailer, metal or paper giving the weight of the trailer and tare weight. Tom
Hey Tom, there is a tag but does not list the weight of the trailer only the max weight allowed. I won't be able to get out to get the RPMs till next week. I think I might move the trolling motor battery up to where the 2 house batteries are and remove them.

Re: Heavier GF16 having trouble getting on plane

Posted: Wed Sep 21, 2022 9:25 am
by pee wee
If you know the model number of the trailer, there is a way to look at archived Academy Sport pages and look it up there. I'm no expert, but I've gone to archive.org and used their "wayback machine" (I think they got the name from the old Rocky and Bullwinkle/Peabody's Improbable History cartoon!) to look at old web pages.

Re: Heavier GF16 having trouble getting on plane

Posted: Wed Sep 21, 2022 3:43 pm
by TomW1
AndrewS wrote: Tue Sep 20, 2022 9:25 pm
TomW1 wrote: Sun Sep 18, 2022 11:53 am Andrew there should be a tag on trailer, metal or paper giving the weight of the trailer and tare weight. Tom
Hey Tom, there is a tag but does not list the weight of the trailer only the max weight allowed. I won't be able to get out to get the RPMs till next week. I think I might move the trolling motor battery up to where the 2 house batteries are and remove them.
Where are the house batteries now if under the center seat don't remove them, they are helping you right now. Tom

Re: Heavier GF16 having trouble getting on plane

Posted: Thu Sep 22, 2022 7:48 am
by OlivierP
Definitely moving weight forward is the way to go.

I have a GF14 (about the same weight as the GF16) with a trolling motor and big lead battery on/under the front deck ; I have installed remote controls and sit on the middle bench. It goes on plane instantly with a 9.9hp. Not fast (16kts top) but no issue with going on plane.

Re: Heavier GF16 having trouble getting on plane

Posted: Sun Oct 02, 2022 2:26 pm
by AndrewS
boat batteries.jpg
I was finally able to make it out today with the tach. The first time I went WOT I was able to get on plane but not any time after that. My RPMs were at 5100-5300.

I was thinking of moving the trolling motor battery up where the house batteries are and just using the one battery for everything. Up front in the bow I have 2 3gal gas tanks.

Re: Heavier GF16 having trouble getting on plane

Posted: Sun Oct 02, 2022 2:33 pm
by fallguy1000
Well, there you have it. You are not achieving wide open throttle, time for a new prop!

Get the right engine specs, but I see 5700-6200 for the current Merc 2022 four stroke 20 hp engine.

So if you only got to 5300; you are at least 500 rpms away from spec. This means you are running about 10% below rated power, or if you push to 6000, 700 more rpms divided by 5300 is almost 15%! Once up and over the hump; you might even be able to back down a bit.

But the engine is a bit undersized in any final analysis.

Re: Heavier GF16 having trouble getting on plane

Posted: Sun Oct 02, 2022 2:33 pm
by OlivierP
If you haven't already, try sitting on the middle bench with a long tiller extension. On a small boat the pilot is the first weight to move around...

Re: Heavier GF16 having trouble getting on plane

Posted: Mon Oct 03, 2022 3:00 pm
by AndrewS
OlivierP wrote: Sun Oct 02, 2022 2:33 pm If you haven't already, try sitting on the middle bench with a long tiller extension. On a small boat the pilot is the first weight to move around...
That won't work because the front seat is further forward because I added more storage there.

Re: Heavier GF16 having trouble getting on plane

Posted: Mon Oct 03, 2022 4:07 pm
by fallguy1000
Surprised TomW has not said anything here, yet. He ought to be able to advise the perfect prop.

Re: Heavier GF16 having trouble getting on plane

Posted: Tue Oct 04, 2022 12:02 pm
by VT_Jeff
Looking at the scum-line in this pic, appears to me that the back-end is sunk pretty deep, esp compared to where you painted the waterline. Does that scum-line accurately represent where it's sitting before you twist the throttle?

https://photos.google.com/share/AF1QipN ... RWTnlqZWlB

Re: Heavier GF16 having trouble getting on plane

Posted: Tue Oct 04, 2022 9:46 pm
by TomW1
fallguy1000 wrote: Sun Oct 02, 2022 2:33 pm Well, there you have it. You are not achieving wide open throttle, time for a new prop!

Get the right engine specs, but I see 5700-6200 for the current Merc 2022 four stroke 20 hp engine.

So if you only got to 5300; you are at least 500 rpms away from spec. This means you are running about 10% below rated power, or if you push to 6000, 700 more rpms divided by 5300 is almost 15%! Once up and over the hump; you might even be able to back down a bit.

But the engine is a bit undersized in any final analysis.
Andrew I am finally home from vacation and am catching up on all my Bateau posts. As fallguy points out you are way out of spec with your Mercury 20HP motor but 4,000rpm's not 500. We need to get you a prop that will turn at least 6000-6100rpm's with your normal load 5700rpm's is the absolute minimum you should be running. So, if you are only turning 5300rpm's now we need to reduce the weight of the boat and pitch of your prop a lot. Please send me diameter and pitch of your current prop so that I can work on some prop numbers.

Also since you have weighed the boat let't go ahead and run the full calculations for the whole boat.

Weight of boat as weighed
Fuel in tanks when weighed when full
Water in bait tank or live wells if any at 8.3lbs per gal
People and passenger weight
Cooler and ice weight
Fishing gear and tackle weight, throwing nets.
Any other gear not onboard when weighed.

Total weight=

Confirm motor iis Mercury 20HP EFI 2020

Tom

EDIT: Had brain freeze the 4000rpm's should be 400. May not need to alter weight of boat.

Re: Heavier GF16 having trouble getting on plane

Posted: Thu Oct 06, 2022 12:43 pm
by TomW1
Andrew disregard what I wrote above, I had brain freeze. From 5300 to 6200rpm's is 900rpm's. 6200 is your motors max rpm's. So, at roughly 200-250rpm's per inch of pitch you need to reduce your props pitch from the 10 pitch you have to a 6 pitch. A 7 pitch would work also but rpms would be a little lower 5900 or so. You can still let me run the full calculation to be sure though.

Regards, Tom

Re: Heavier GF16 having trouble getting on plane

Posted: Thu Oct 06, 2022 1:25 pm
by fallguy1000
TomW1 wrote: Thu Oct 06, 2022 12:43 pm Andrew disregard what I wrote above, I had brain freeze. From 5300 to 6200rpm's is 900rpm's. 6200 is your motors max rpm's. So, at roughly 200-250rpm's per inch of pitch you need to reduce your props pitch from the 10 pitch you have to a 6 pitch. A 7 pitch would work also but rpms would be a little lower 5900 or so. You can still let me run the full calculation to be sure though.

Regards, Tom

Isn't there a top speed problem dropping all the way to 6 pitch?

Good post Tom...just wonder if you can guess at top speed with the 6 vs the 7.

If you now he can't plane now and the rpms, probably can determine the weight by backing into it.. no?

Re: Heavier GF16 having trouble getting on plane

Posted: Thu Oct 06, 2022 1:44 pm
by Fuzz
I would really like to see a picture of the mounted motor. Feels like something is wrong and it is not just the prop.

Re: Heavier GF16 having trouble getting on plane

Posted: Thu Oct 06, 2022 4:31 pm
by TomW1
fallguy1000 wrote: Thu Oct 06, 2022 1:25 pm
TomW1 wrote: Thu Oct 06, 2022 12:43 pm Andrew disregard what I wrote above, I had brain freeze. From 5300 to 6200rpm's is 900rpm's. 6200 is your motors max rpm's. So, at roughly 200-250rpm's per inch of pitch you need to reduce your props pitch from the 10 pitch you have to a 6 pitch. A 7 pitch would work also but rpms would be a little lower 5900 or so. You can still let me run the full calculation to be sure though.

Regards, Tom

Isn't there a top speed problem dropping all the way to 6 pitch?

Good post Tom...just wonder if you can guess at top speed with the 6 vs the 7.

If you now he can't plane now and the rpms, probably can determine the weight by backing into it. no?
Dan that is why I would like to run the full calculation. Right now, all we know is the weight of the boat, and what he had on it which we do not know? Look at the info I ask for in the calculation, I need as detailed as possible list of the weight as the boat leaves the dock. Normally the reduction in pitch and the increase in rpm's offset so that speed does not change. Tom

Re: Heavier GF16 having trouble getting on plane

Posted: Thu Oct 06, 2022 4:53 pm
by TomW1
Fuzz wrote: Thu Oct 06, 2022 1:44 pm I would really like to see a picture of the mounted motor. Feels like something is wrong and it is not just the prop.
Fuzz that may also be a problem, but I am giving Andrew the benefit of the doubt here and assume he has the ventilation plate mounted even with the bottom of the transom. His main problem is weight distribution and prop size. Andrew if your ventilation plate is not even with the bottom of the transom that is the first adjustment you need to make. Tom

Re: Heavier GF16 having trouble getting on plane

Posted: Thu Oct 06, 2022 5:14 pm
by OrangeQuest
If weight is the one factor that the boat is not getting on plane easy, then a reduction in pitch will also help it get on top of the water easier.

Re: Heavier GF16 having trouble getting on plane

Posted: Fri Oct 07, 2022 12:18 pm
by fallguy1000
About the only other possibility is if he is too low with the prop, but he would still be able to get rpms up to wot with lower pitch..if he is up too high; he'd spin out on a turn

Re: Heavier GF16 having trouble getting on plane

Posted: Fri Oct 07, 2022 2:52 pm
by TomW1
fallguy1000 wrote: Fri Oct 07, 2022 12:18 pm About the only other possibility is if he is too low with the prop, but he would still be able to get rpms up to wot with lower pitch..if he is up too high; he'd spin out on a turn
Dan looking at his pictures I think that he may be a little too low, it is not a great pic to be able to tell so I advised him to raise the motor if the ventilation plate is not even with the bottom of the transom. Tom

Re: Heavier GF16 having trouble getting on plane

Posted: Fri Oct 07, 2022 7:24 pm
by piperdown
TomW1 wrote: Fri Oct 07, 2022 2:52 pm
fallguy1000 wrote: Fri Oct 07, 2022 12:18 pm About the only other possibility is if he is too low with the prop, but he would still be able to get rpms up to wot with lower pitch..if he is up too high; he'd spin out on a turn
Dan looking at his pictures I think that he may be a little too low, it is not a great pic to be able to tell so I advised him to raise the motor if the ventilation plate is not even with the bottom of the transom. Tom
It appears that vent plate is a couple of inches below the bottom of the transom. I enlarged the pic a bit but it's definitely below the bottom of the transom.

Re: Heavier GF16 having trouble getting on plane

Posted: Fri Oct 07, 2022 9:29 pm
by TomW1
piperdown wrote: Fri Oct 07, 2022 7:24 pm
TomW1 wrote: Fri Oct 07, 2022 2:52 pm
fallguy1000 wrote: Fri Oct 07, 2022 12:18 pm About the only other possibility is if he is too low with the prop, but he would still be able to get rpms up to wot with lower pitch..if he is up too high; he'd spin out on a turn
Dan looking at his pictures I think that he may be a little too low, it is not a great pic to be able to tell so I advised him to raise the motor if the ventilation plate is not even with the bottom of the transom. Tom
It appears that vent plate is a couple of inches below the bottom of the transom. I enlarged the pic a bit but it's definitely below the bottom of the transom.
Yea, it looked low, but pics can be deceiving. Tom

Re: Heavier GF16 having trouble getting on plane

Posted: Mon Oct 10, 2022 7:58 am
by AndrewS
Hey guys sorry for the delay. I think the vent plate is pretty even with the bottom, but I'll get another straight edge and try and take a better photo for you. To be honest I haven't checked if that is where the boat is sitting loaded down. I will be gone on a trip till next weekend. I think the prop diameter is 9.75. Measured from the center out to one tip x2.

Weight of boat as weighed: 840
Fuel in tanks when weighed when ful: it had 3gal on it when weighed, 6 gal total capacity.
Water in bait tank or live wells if any at 8.3lbs per gal: N/A
People and passenger weight 575
Cooler and ice weight: 30
Fishing gear and tackle weight, throwing nets. 3 Rods and small tackle box.
Any other gear not onboard when weighed.

Total weight= ~1,493lbs

Re: Heavier GF16 having trouble getting on plane

Posted: Mon Oct 10, 2022 12:56 pm
by TomW1
Thank you Andrew I will work up the calculations using a 20HP Mercury motor and see what we get. If you could tell me exactly which Mercury 20HP you have, I will assume it is the standard 4 Stroke otherwise. Tom

Re: Heavier GF16 having trouble getting on plane

Posted: Mon Oct 10, 2022 2:32 pm
by AndrewS
Hey Tom thanks! I think it's this one: 2021 Mercury 20 HP EFI 20MLH

Re: Heavier GF16 having trouble getting on plane

Posted: Mon Oct 10, 2022 2:52 pm
by TomW1
AndrewS wrote: Mon Oct 10, 2022 2:32 pm Hey Tom thanks! I think it's this one: 2021 Mercury 20 HP EFI 20MLH
Great that's the one I looked up and have the specs on. Tom

Re: Heavier GF16 having trouble getting on plane

Posted: Mon Oct 10, 2022 6:59 pm
by richardm
Posting my experience with my GF-12 for what it is worth:

Engine is a 2017 Mercury 9.9 four stroke, stock 8.9 x 8.5 prop. At 622 lbs displacement (one person) at WOT, 5800 rpm and 20.5 mph by gps. At 825 lbs displacement (two people) at WOT, 4500 rpm and 10.8 mph.

I added a doel fin and moved down to 8.9 x 6.5 prop. With this configuration, at 825 lbs, 5800 rpm and 12.4 mph. With one person WOT over rev'ed, but backing off to 5800 rpm got 16.3 mph.

With respect to AndrewS's situation, my opinion is:
1. going down in prop pitch will help, but not make him happy.
2. rocker may be a factor if he followed the plans and building notes.
3. a 20hp motor is too small for a boat displacing almost 1500 lbs.

Re: Heavier GF16 having trouble getting on plane

Posted: Tue Oct 11, 2022 6:05 pm
by TomW1
Andrew here is what I came up with. First you need to balance your weight so the boat is floating even on her lines. That may mean moving the batteries under the front casting deck, and you sitting on the middle seat and using a tiller extension. But after doing that your motor will give you a top speed in the very low 20's 20-22 per my calculator. A propellor of 9.75 x 8 pitch would be appropriate. What you are doing right now is dragging all that weight in the stern which keeps you from getting on plane.

But the most important thing is to get your boat balanced. Also, you need to get the ventilation plate within an 1/2" of the transom bottom.

Please feel free to ask any questions.

Tom

Re: Heavier GF16 having trouble getting on plane

Posted: Wed Oct 12, 2022 12:12 am
by fishgitr
Raise that jack plate up. You probably have 3-4" of play that you can go up. It looks manual and you can play with it a 1/2" at a time until you get blowout turning. It can't be even with the bottom bc you have a set back.. it needs to come up.. distribute the weight..

Why do you need two house batteries and trolling motor battery? Thats 120 pounds you can reduce there. Trolling motor up front with battery.