View Full Version : How Do I Determine the Correct Prop Size/Pitch?
12-05-2008, 08:38 AM
My boat seems to be over-propped, but I am unsure how to determine what the right size should be. Is there a prop calculation formula? My boat is 46ft Chesapeake Deadrise, wooden hull, Net 8 Tons on the documentation, MarinePower 350 rated for 315hp@4400rpm, Velvet Drive 1.91:1, current prop is 4 blade 20x20. The engine will not rev beyond 3600rpm. Ideally I would like the boat to remain on plane at 3200rpm, which is MarinePower's recommended "optimal" rpm. With this hull/configuration I am obviously less concerned with speed/economy, just want to run the engine optimally to achieve the most longevity.
A good prop shop should have the ability to evaluate the prop needed for your boat. They will need to know the LWL, BWL, d, Displ., # of props, HP, gear ratio, & hull type (planing, semi-planing, displacement).
Or you can patronize your nearest friendly boat designer for the information.
12-05-2008, 10:06 AM
Has the prop been re-pitched in the past? Is it a bronze prop?
Hopefully, you'll get lucky and be able to pull an inch or two of pitch out of it and end up where you'd like to be.
Sometimes the programs can be off a bit. That's when experience and the black art of picking the proper pitch
12-05-2008, 11:08 AM
If your boat is a trdeitional Chesapeake Deadrise the hull form is usually not a true planning hull form. It would typically be designated a fast displacement hull or possibly a slow planning hull. Mine (1966 wood hull, 403 cu.in. Olds engine) will not plane unless I'm throwing a lot of horses and fuel at it. So it just isn't worth the engine wear and expense. It does move along easily and economically at 10 knots though. Does it plan now at the 3600 RPM you can attain? I suspect not. Anyway it does appear that you are over propped. As MMD suggested, get all of the data you can possibly get and talk to a good prop shop.
12-05-2008, 12:05 PM
I was sitting here having lunch when I came up with a few other thoughts. Remember these thoughts are worth what your paid for them. However, I am very familiar with the typical deadrise hull, and somewhat familiar with your engine. Anyway here goes. I was always told that a gas engine should be run at 70% of its max. RPM, In your current case this would be 2520 PRM (3600 X .7). If you repropped it so you could reach 4400 RPM you should be running it at 3080 RPM. If you look at the HP vs. RPM curve for your engine I think that you'll see that it just isn't producing enough horses at that RPM to plane.
Bottom line is a 315 hp gas engine seems at little weak to plane a 46 ft. boat without beating the hell out of the engine. I know of a couple of deadrise workboats of your approx. size around here with 350s in them and I never see them on a plane, which means either they can't or it is too hard on the engine and too expensive for fuel to do it. Now if you had a 315 hp diesel where you could run it at 90% of max RPM all day long, it would be a different story.
12-05-2008, 01:26 PM
From model plane experiments, where props are relatively cheap to buy and modify...
For best peak speed,
Reduce prop pitch to let the engine rev to achieve more power.
If this allows the engine to rev but speed doesn't improve, increase the diameter.
In your case, the engine is getting to 3600 rpm and the peak HP is higher (4400 rpm), so I'd begin by reducing pitch a bit.
On the other hand, it sounds like you're really trying to improve your cruise speed. If you're willing to give up on top end, then increase pitch and/or diameter at bit. If the engine won't reach 3200 any more, then you've gone too far.
12-05-2008, 03:11 PM
I would be suspicious of an advertised 315 continuous duty horsepower from a 350 cu. in engine.
That’s about what you might expect from an 855 cu. in Cummins.
12-05-2008, 03:25 PM
Thanks Dale. Yes "plane" is a unique and strange term with Chesapeake Bay deadrise boats, especially old ones and this boat was built in 1949. When I refer to "plane" I am only speaking about flat water in my wake as opposed to pulling a 3+ foot swell. Depending on tide/wind, yes I have achieved flat water wake at 3200 running 15 kts. I generally do run her at 2500-2700 rpm, 10kts. When I was quizzing the folks at MarinePower about optimal rpm for engine life, they mentioned 4400 as max, 3200 as optimal. Since I am not revving to 4400 is why I began researching what the correct prop size/pitch should be.
12-05-2008, 03:30 PM
Canoeyawl - The 350 cu.in. gas engines by Crusader and Marine Power are rated 315 hp @ 4400 rpm. You don't want to run them there, at least not for long.
BTW yesterday I delivered a fiberglass express cruiser with twin Detriot 671 engines in it. A 671 is 426 cu.inches and they were rated 450 hp each. True, they were turbos. But the point is that it is no problem to get 1 hp per cu.in with gas or diesel.
12-05-2008, 04:03 PM
bboykin - If you can get 15 @ 3200 rpm knots out of it I think that is pretty good. I don't think my boat ever saw anything above 14. True it is a smaller boat (30 ft.), but the old Olds 403 (circa 1978) is ony rated 265 hp at 4000 rpm. Decreasing the pitch will probably result in having to run the engine at a higher RPM to do that. That may not be a bad thing. It depends on how "hard" the engine is working at a particular rpm. That is where a good prop. shop can give better advice than I can.
I know the gas engine manufacturers say to run the engines at 3000-3200 rpm. But remember these engines are really just car engines. What rpm is a car with a good old chevey 350 running at when the car is cruising down the highway at 65 mph. Probably 2300 or so. That is where I like to run these engines for maximum life.
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