Boat Builder Central - Builder Forums

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.

Surprised TomW has not said anything here, yet. He ought to be able to advise the perfect prop.

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

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.

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

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?

I would really like to see a picture of the mounted motor. Feels like something is wrong and it is not just the prop.

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

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

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.