How do I set up the hieght of the prop relative to the bottom of the boat?

[proaudioguy]

I have a V4 140 Evinrude 25". I have a Stratos 200DC 20ft dual console. It's not a bass boat. It's more like a fishing boat/ bow rider. The transom is cut out, the engine mount sits high with a large gap at the top where it might normally be attached to the top of the transom. I'm thinking perhaps this particular boat should have a 20" instead of a 25" but I just don't know. Looks like the guy my dad bought the boat from did an OK job getting it close, but I want to get it RIGHT.

I went through a prop wizard on another site (don't remember where) and it said I needed a 13 1/4" x 17" or a 13 1/4" x 19" prop. I want to be able to pull a skier or two or go out fishing for the day. In either case the prop wizard says use the 13 1/4 19". The 17" is only recommended for the casual cruising scenario where the additional load is not there. It currently has a 15" x 17". How far off is that really? Props are really expensive apparently. I don't know what to buy. At wide open throttle we do have a bit of cavitation (I think that's the right word for what is happening). I back the throttle down once it's on plane, and it stops gurgling back there. Then I can push it up a little more. Before this week I had not been able to get it on plane much. I am gradually fixing all the issues with this boat and learning a lot in the process. I also can't figure out where to trim it and when. My nick should be NOOB!

 

[Silvertip]

Re: How do I set up the hieght of the prop relative to the bottom of the boat?

You gave us no rpm numbers to work with. Make some wide open throttle runs with what you consider and average load and note the rpm you observe on the tach. Be sure to test both props. Next, the "casual cruising info for a 17P is wrong. A 17P prop is what is needed for pulling water toys and heavy loads. A 19P prop is a cruiser and light load prop. That assumes of course that one of the two props is correct.

A 25 inch leg on that boat seems long but you can tell that by looking at the boat from the rear. The AV (antiventilation plate) which is the flat plate just above the prop as a minimum should be flush with the bottom of the boat or one inch above it. I'm guessing but I suspect it currently sits well below the keel. To change the height you unbolt the engine and move it up or down one hole and bolt it up again. Test and repeat until you get it right. One inch up is a starting point and may or may not be the optimum setting.

Until you provide some RPM numbers from both props we can't tell you which one is correct or if either one is correct. I would not spend money on props until you get the engine height situation squared away.

Last -- if you do determine you need a different prop be very aware that what may appear to be identical props but from two different manufacturers, they may perform and usually do perform differently. If you have an OMC prop now, look for another OMC prop of the same series but with the pitch you need. Then and only then can you reasonably predict what the change will do.

 

[proaudioguy]

Re: How do I set up the hieght of the prop relative to the bottom of the boat?

I don't even have a working tach on this boat and didn't know I needed those numbers. I also only have the prop that is already on the engine so I have nothing to compare with. I have no idea what kind of prop it is. If I knew what to buy, I would buy it. I thought a 13 1/4" x 17" prop would be the same regardless of manufacturer. You are right, I had it backwards on the prop thing. It says 21" for a completely empty boat with 2 people, up to a 15" for 6 people lots of gear, loaded live wells and lots of gas. 17"-19' are the compromises. They all say 13.25" while the prop I have is a 15". Assuming the wizard is correct, I might need to own 2 props. I have no connections for borrowing props to try like some folks on the board. I'm very new to the boating thing. My parents always had boats, and so if I wanted to go boating I went with them.

The info you gave on the height is good. The engine mount to the transom is very high with space above the transom, about 2.25" and it's the lowest hole on the engine mount meaning the engine can't get any HIGHER than it already is. Perhaps I should take a picture. It may be fine, but it looks really odd. I plan to extend the engine with an extension bracket and want to make sure the brackets out there will work, but the engine may be too high.
The anti-ventilation plate is almost dead even with the bottom center line of the boat. I think if I had a 20", that would push that 5 inches above, but then the engine mount could be lowered 2.25 inches by changing to a high hole. That would put the plate close to where you said it should be. If that is the case, I guess the question is how to proceed? I'm definitely getting some air to the prop at WOT. Perhaps the transom is just too low?

I appreciate your reply, very educational. I really would have thought the anti-ventilation plate should have been lower into the water.

http://boatmartusa.com/marinemall/p...ID=9EjVXY1617jN1650JMVP1Z7E33Il1815VE1584MNyA

 

[Willyclay]

Re: How do I set up the hieght of the prop relative to the bottom of the boat?

Perhaps I should take a picture.

I do better with pictures. The link below is the Mercury "show & tell" on this subject.

http://continuouswave.com/whaler/reference/engineMountingHeight.html

Silvertip and other forum experts can help you solve the problem if you follow their advice.
Good luck!

 

[proaudioguy]

Re: How do I set up the hieght of the prop relative to the bottom of the boat?

Hey guys, lots of learning on my end tonight. Thank you. I'm starting to think the motor is very close to where it should be and the ventilation that I get at full throttle might be heavily influenced by the fact that the prop is 1.75" larger diameter than recommended. That's about .875" further out of the water than the recommended 13.25" diameter. Your thoughts?

 

[sublauxation]

Re: How do I set up the hieght of the prop relative to the bottom of the boat?

I'm fighting this same battle, propping is a blend of art and science.

I second what Silvertip said. First thing is to find a tach, you have a lot invested in that motor, finding a good evinrude tach/system check gauge combo on Ebay for $50 or less should be pretty easy. That gives you a basis for making all future changes.

Next I'd say weigh the pros and cons of AL vs SS because they will act differently, and as Silvertip said, even different brands of the same size prop will act different, some will run better at different heights also, you can weigh the venting vs rpms when you get a tach.

Some shops will let you try different props as long as you buy one from them and dont bang it up.

 

[Silvertip]

Re: How do I set up the hieght of the prop relative to the bottom of the boat?

Without a tach you have no idea whether the prop you have is best or whether something else would work better. Remember the word "design". Prop manufacturers have different designs even though diameter and pitch may be the same as some other manufacturer's "design". Blade shape, blade size, rake, material, cupping, are just a few of those characteristics so diameter and pitch are only part of the design process. If you are on a budget, go to Greatlakesskipper.com (Great Lakes Skipper) to search for tachs. I've seen them as low as $30 bucks. If the application charts show that the prop diameter for your engine 13-1/2 then you currently have the wrong prop. Too many folks simply slap a larger diameter prop on a boat with no comprehension of what the outcome will be. It is equally puzzling why they do that since without a tach one has no idea if the engine is over reving, lugging or spot on. Whatever you do, don't do two or three things at once. Change one thing -- then test. Changing more than one thing makes it difficult to determine what went wrong if performance really decreases.

 

[proaudioguy]

Re: How do I set up the hieght of the prop relative to the bottom of the boat?

Great, where can I find the recommended WOT RPM for my engine? Ihave looked for it with no luck. Just not looking in the right place I guess.