B31 Prop Sizing

[JoeBro]

I purchased a B31 Sportfish a little over a year ago. It came with 310 HP Diesels and had 18x18 3 blades on it with 1.5:1 ZFs.
The motors maxed out at 3600 rpm and she would only do about 28 knots with the throttles pegged. Under propped.
When I hauled her I switched out the original props with 19x22 (light cup) 3 blade dynajets and she would do 32+ but the motors only got to 3300rpm. Over propped.
I pulled the 19x22s off when I hauled her in May and put the 18x18s back on. I am going to take the 19x22s to the prop shop and get them re-pitched.
Before I do that, I thought I would ask you for your opinions. I'm thinking 19x20 is the right pitch and should bring the rpms up to 3600 rpm at 32 kts.
Any recommendations?

[Snipe]

Joe what engines are in the boat? Yanmar? That could help the guys with a similar setup try to help you.

[JoeBro]

2018 Peninsular 400TA 6.5L diesels max 310 HP at 3600 rpm

[Tony Meola]

I believe Brewster used to have the Peninuilar in his boat. Unfortunately he has not posted in a while.

If the engines are rated at 3600 HP, you probably want to come in around 3650 to make up for when the boat is fully loaded. Taking out some pitch should work. Usually you will find most of the boats here running Yanmars or Cummins and the props are close to square. If you take out pitch and find the RPM's get too high. Try and use cup to make up some of the difference.

Hopefully Bruce will see this post and he might be better able to answer your question.

[Carl]

Tony- I believe Brewster had Perkins prior to the Yanmars. Penisular are GM motors.

JoeBro- Rule of thumb, an inch of pitch removed should add 200rpm to your top end.

So 2" removed should add 400rpm giving you 3700 WOT...although curious why the 18 x 18 didn't go over 3600 at WOT. Have you photo tached the motors to confirm your tachs are correct?


The Thing about "rule of thumb" is words like Should and Could come into play.

I'd feel better being slightly underpropped hitting 3700 than being overpropped...plus if confirmed your still a tad underpropped you could always go back and have them add a slight cup for a perfect 3650rpm. Or go with 1-1/2" pitch reduction...but if not enough you need another pitch change.

Best thing...speak with your prop shop, a good one will work with you helping to dial you in.

[JoeBro]

Thanks Carl and Tony.
A couple answers to your questions.
Yes I have a photo tach and correlated my dash tach readings with it it.
Why are the engines not going over 3600 rpm?
1) Because that’s where the max hp rating is at. In theory it is the energy required to push the boat over the water at max speed. To go faster you would need more hp and to get more rpm you would need an engine that has a max rating over 3600 rpm.
2) sometimes diesels are governed to not exceed max rpm. But these AMGs don’t have governors.
Rules of thumb - I’ve heard the 200 rpm for an inch of pitch before. Also some folks say thats for a single engine but with twins you use 70% of that.
I have contacted a bunch of propeller shops and they have come back all over the place. Manufacturers are a good resource as well. Michigan Wheel does calcs for you on request and there are free propeller sizing calculators on the internet. Vicprop has the best IMO. I think in the end prop sizing is an iterative process and you just have to keep trying until you get it dialed in.
I’m going to the prop shop today and will get their recommendation. I bet it will be 19x20.

[Carl]

Why are the engines not going over 3600 rpm?
1) Because that’s where the max hp rating is at. In theory it is the energy required to push the boat over the water at max speed. To go faster you would need more hp and to get more rpm you would need an engine that has a max rating over 3600 rpm.
2) sometimes diesels are governed to not exceed max rpm. But these AMGs don’t have governors.

Oh wait I read it wrong, what is the rated WOT rpm you should hit under load, I assumed it was the 3600.

[scenarioL113]

It does not matter what engine you have, you need to hit your Rated RPM plust 50-100 rpm more when you are loaded. This will give you the best results and keep your engines safe from lugging.
Yes, according to your troubleshooting it looks like you will end up in the 19X20 range on your props to get it right.