Heavier GF16 having trouble getting on plane

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fallguy1000
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Re: Heavier GF16 having trouble getting on plane

Post 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
My boat build is here -------->

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TomW1
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Re: Heavier GF16 having trouble getting on plane

Post 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
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

piperdown
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Re: Heavier GF16 having trouble getting on plane

Post 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.
Eric (aka, piperdown)

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TomW1
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Re: Heavier GF16 having trouble getting on plane

Post 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
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

AndrewS
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Re: Heavier GF16 having trouble getting on plane

Post 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

TomW1
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Re: Heavier GF16 having trouble getting on plane

Post 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
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

AndrewS
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Re: Heavier GF16 having trouble getting on plane

Post by AndrewS »

Hey Tom thanks! I think it's this one: 2021 Mercury 20 HP EFI 20MLH

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Re: Heavier GF16 having trouble getting on plane

Post 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
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

richardm
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Re: Heavier GF16 having trouble getting on plane

Post 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.

TomW1
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Re: Heavier GF16 having trouble getting on plane

Post 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
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

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