Sunrunner 3700LE correct Propeller size

[White Shadow]

Hello everyone,
I would like to find out what the correct propeller size is for my Sunrunner 3700LE, 2003 model
It is fitted with 2 x 5.0 Litre MPI engines
With Mercruiser Bravo 2 Legs.
Unfortunately all marking on the current props are unreadable and I would just like to know what was fitted when it was new.
It would be greatly appreciated if someone could tell the correct diameter and pitch as there are so many variants and it's a lot of money to out lay for trial and error.
Thank you

[tkeithlu]

Welcome to the forum, White Shadow.

A direct answer to your question may be dependent on finding an owner with that exact boat, but there are other approaches.

An experienced prop shop may be able to estimate what you need.

How is the present prop performing? You are looking for a maximum RPM below the red line for the engine, and propulsion at hull speed with perhaps a bit below that. Diameter is easily measured, and I think I can explain the tricky task of measuring pitch. Or, take your prop to the shop, and tell them whether the engine over-revs or bogs down. If your water line length is 35 ft, your hull speed is 1.34 times the square root of 35, or 7.9 knots.

[tkeithlu]

Whoa, sorry, that's a planing hull. Still looking for a top end on the engine a bit below red line, but a comfortable plane below that.

[White Shadow]

Thanks for the reply tkeithlu,
Unfortunately the props are in such bad condition i can't determine the diameter or even test drive the boat to accurately assess the WOT rpm.

[Squanderbucks]

Wrong wrong wrong. On a planing boat especially but any other as well you do not want to prop it so it can not reach AT Least top rated RPM and preferably 100 to 150 RPM above that when the boat is fully loaded with your expected fuel people and provisions.

Anything less will result in your engine being overloaded and reduce engine life and increase fuel burn.

That is not to say you would actually be running at that speed regularly but this would insure engine load at any given RPM would be proper. Anything less and you will reduce engine life.

Also this allows good performance when the not so clean bottom takes away performance which equates to more unintentional load.

Propping and engine so it turns anything less than top RPM is a sure engine killer.

Think about running your boat when the bottom has begun to foul. Greatly reduces performance. Fuel use gas goes up and speed goes down. When the boots and gear are not clean and you run at your “normal” RPM there is considerably more load on the engine. It’s not all about the RPM it’s really the load you want to consider.

[tkeithlu]

That sounds like under-proping to me. Where I used to work and from the dealer that proped my current small boat, we aimed at 5,500-6,000 RPM top and then ran them at 5,000. Less prop would mean that they never reached full power. I do follow your logic, but am confused - has all that experience - at least 20 boats and 25 years - been wrong?

[BillKny]

Originally Posted by tkeithlu

An experienced prop shop may be able to estimate what you need.

Any prop shop can easily measure EXACTLY what you have.

You can do it yourself if you do a Google search on how-to, although with the tools it is a bit fussy.

[White Shadow]

I have finally found out what the original prop size is for my Sunrunner 3700LE.
The part Number is 48 18616 21P and its a
17¾ diameter x 21 Pitch.
I hope this info is of help to anyone else who may need it.

[rsn48]

I just gave this answer about one hour ago in the Trawler forum, so I'll just copy and paste to here:

Shhhh!! Been there, done that. There is prop software on the net that can help you out. Remember to not input your boats dry weight. No one wants to recommend a prop for you because they know all recommendations are estimates. I bought a Merc engine and leg and they didn't want to recommend a prop. But they do have prop software I used. I called prop shops in BC and Washington state and they would point in the right direction but not commit. Some shops will let you purchase and return as long as you are purchasing the next one from them. There probably will be a small percentage taken off on the returned product as they can only resell it as used.

[Squanderbucks]

A lot of the conventional wisdom thrown about is from seat of the pants I go passed down from person to person and not born out by hard data that the engine manufacturers diligently work out and after seeing the results of all the things owners try out on their rigs.

Engine life is all about load. Manufacturers make up fuel burn curves and show what you should be burning at any given rpm. If you prop your boat do that it does not achieve its top rpm it will be overloaded at all rpms. May not hurt much at trolling speeds but at cruise and over you are wearing it out regardless if the RPM and burning excessive fuel.

But because we really never know and if the boat seems “right” a d it doesn’t seem like a real hog then everything is OK. No matter the hull type the engine doesn’t care it just wants to meets it’s designed performance curves. Different boats may run at different mph but the engine doesn’t care. It only cares about load.
Propping the engine do its top rpm is lower than the rated maximum “seems” intuitive that you are working the engine less but the opposite is true. The bigger prop means a bigger load at any given speed. It’s like pulling an anchor out behind the boat at all times.

Go to Tony’s tips at SBMAR.com and look up the stuff about propping boats. Also check out the stuff about marine age vs actual age. Great starts to understand this stuff.