Jon boat prop question

[Peedeecat]

Hello folks. I have a 16 foot aluminum Seaark with remote steering with a 95 40 hp Mercury 4 cylinder 2 stroke with t&t. It came to me with an 11 inch 12 pitch prop. The motor has 125/130 compression on all four cylinders. It runs strong and smoothly. My transom is 21 inches and the motor is a long shaft with the cavitation plate set even with the bottom of the boat. It comes out of the hole like a rocket but will only achieve 22 mph trimmed out. I do not have a tach, but I have been around boats for most of my 65 years. To the ear, the rpms seem pretty good. I tried a friends 11 3/4 x 16 prop and achieved 28 mph but was sluggish out of the hole. I would like to achieve at least 30 mph. I realize these four cylinder Mercs are not speed demons, nor will they keep up with the 40 Yamahas, but I think I'm missing something related to my prop. Heck the 25 Evinrude with a 10/15 prop would run 25 mph on it. Any suggestions would be greatly appreciated. Btw, if I raise the motor up a hole, it starts cavitating. Scratching my head in SC. I do realize the first suggestion will be to get a tach. Beyond that, what? Can't afford to start buying random props to try. Thanks in advance!

 

[jimmbo]

Without the RPMs you don't have a reference point to start from. Get a Tach, 99.999% of Boaters couldn't correctly guess an Engines RPM.

 

[ahicks]

Sounds like you already know the answer. Buy a tach! UNLESS, you've gotten your speed reading using the boat's speedo only, without confirming it via GPS. If using the boat's speedo, you could be way off there as well.

 

[Peedeecat]

Using GPS speedo

 

[Peedeecat]

Was basically hoping someone here had a similar boat and setup that could give me a ball park starting point pertaining to prop diameter and pitch. I do intend to hook up a tach. I can remove the tach from my 200 Merc Black Max if I have to.

 

[WesNewell]

Put your engine down in line with the hull and take a photo similar to this to check engine mount height. An engine mounted too low can cause up to 20% loss of speed and fuel economy. Assuming your boats dry weight is 500#, you should be able to get about 32 mph out of it with your engine with everything setup properly.

 

[anzomcik]

Aside from the needing GPS speed, and a tach, we are only guessing

But i have a console 14' with a mariner 40hp, it was a full floor, casting deck....it was heavy for a 14'. i was at 32-34mph. trying to remember but i think a 13 pitch

 

[Texasmark]

Here's an idea of a limit of your expected performance. Engine is 2:1 gear box, 12" pitch prop, max rated rpm 5500 specs! For a "limit" in the calculation, take prop slip at 0 and see what your max possible speed would be (like a screw turning in a piece of wood):

https://www.mercuryracing.com/prop-slip-calculator/

Prop slip on a John is probably running around 10%, easy planing, flat hull, low drag, PTT a plus:

https://www.mercuryracing.com/prop-slip-calculator/

Assume you are running 10% and with PTT on that boat a very reasonable assumption, at your stated accurate 22 mph you are running:

https://www.mercuryracing.com/prop-slip-calculator/

Sir, I think you are getting all you are going to get out of that setup.

If I copied you correctly, your reference to a 25 Rude, old days 25 hp, like 35 cu in, running 4500 rpms max? I remember 40 Mercs, the original Mark 55 40 hp, from which your engine is a descendant, had a hard time keeping up with those chuggers. When OMC upped the hp to 35 and 40 forget it.

My 2c.

 

[Peedeecat]

I took the tach out of my other boat. Its an oem Mercury tach. Best I can determine she's only turning 4400 rpms. I think I may have to go down in prop diameter and up in pitch. Just don't know how much. Last weekend we did a leak down test on the cylinders and had zero leak down on all 4. We did find a bad plug wire on #3 and replaced it and gained 1 mph. Also noticed a hook in the last two feet of the hull. I'm sure that is also contributing to the slow speed. I'm going to finesse the hook out of it, easy with aluminum. Again my current prop is a stainless 11 diameter with a 12 pitch. With this added info on the tach reading can anyone make any suggestions as to what prop size I should try to get the rpms up?

 

[WesNewell]

Check the tach setting and make sure it's set for the engine you have.

 

[Texasmark]

I took the tach out of my other boat. Its an oem Mercury tach. Best I can determine she's only turning 4400 rpms. I think I may have to go down in prop diameter and up in pitch. Just don't know how much. Last weekend we did a leak down test on the cylinders and had zero leak down on all 4. We did find a bad plug wire on #3 and replaced it and gained 1 mph. Also noticed a hook in the last two feet of the hull. I'm sure that is also contributing to the slow speed. I'm going to finesse the hook out of it, easy with aluminum. Again my current prop is a stainless 11 diameter with a 12 pitch. With this added info on the tach reading can anyone make any suggestions as to what prop size I should try to get the rpms up?

150-200 rpms per inch up or down inverse to pitch change (obviously). If you are running a SS currently it may have cupped blade tips. Allow an increase of 1" in the posted pitch for that "luxury"....which it is. So a 12" would calculate as a 13"P.....not my words, readings from prop mfgrs.

Hook can be a speed problem. It would tend to put the bow down making for plowing and lower speeds, requiring prop to be moved farther away from the transom (tilt-trimmed out farther). Name of the game is to get the hull out of the water.

 

[Peedeecat]

Not having any trouble getting the bow up with the motor trimmed up. The hook can still create a lot of drag though. Was hoping to finesse the aluminum out flat today but just didn't get around to it. I appreciate all the advice and will keep y'all posted on my progress.

 

[jimmbo]

How are you going to brace the Aluminium so it remains true?

 

[Peedeecat]

I am a metal fabricator by trade, and bracing or any other aluminum mods is no problem. I got the hook out of the hull yesterday and will test run it tomorrow, checking the tach reading again. Then, go from there on prop choice.

 

[Texasmark]

I am a metal fabricator by trade, and bracing or any other aluminum mods is no problem. I got the hook out of the hull yesterday and will test run it tomorrow, checking the tach reading again. Then, go from there on prop choice.

"Then, go from there on prop choice." That's what the big guns on here used to say: "Get your setup right and then dial in your prop."