Doing the Prop Math on my Adventure 208

MooseheadDoc

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Still planning my winter and spring projects for my new, old 208. I've been reading a lot of threads about Prop performance and engine RPM performance.

On my few runs after buying the boat and before winterizing I know I am getting low RPMs at WOT.
I knew this will obviously also give me slower speeds, but as I read more about props- if I'm doing my math right- my speed is way slower than it should be at the given RPM, meaning my "prop slip" is huge.

My prop is a stainless 14.25/17. The gear ratio of my Johnson 175 2s is 1.86. At 4300rpm I calculate that should give me a theoretical speed of about 37mph. I'm getting less than 25mph. That's a huge prop slip.

From what I read, many factors will negatively affect both rpm@WOT and prop slip.

For the sake of this post I'll ignore engine performance for 2 reasons. First, I leave those to my mechanic. I don't do engines (really wish I did, sometimes). Second, gaining 500rpm of WOT engine performance alone does little to affect my performance if I don't change the boat characteristics.

So, I plan to do the following as soon as I'm able:
1. Try a new prop. Even thought the prop looks OK, maybe it has a shape or hub issue. Easy enough to try a different prop. If possible, I'll try combinations of lower pitch (for more RPM), and wider diameter (for lower slip). Sounds like it's really trial and error to find the actual sweet spot. Obviously I'll get the best results if I can both increase RPM and reduce slip.
2. check the engine mount height. Just eyeballing the engine it seems to sit low. Once its out of the water I'll check the cavitation plate relative to the bottom. Maybe it needs to come up a peg or two.
3. Add trim tabs. I've read a bunch of pro/con posts here and on other forums. I'm strongly leaning toward adding them.
4. Clean the bottom as well as possible. There is little growth, but lots of rough, old layers of bottom paint. No one near me does soda blasting. I'll have to figure out my options in the spring.
5. I don't think I'm waterlogged. It's hard to evaluate the foam-filled hull sections. The parts I can see or reach are dry. I'll weigh the boat if I have a chance, but since this isn't a variable I can do much about, I'm focusing on everything else first.

Any other thoughts or suggestions? I'd love to hear that I'm missing something obvious.................. JB

Joe
Beaver Cove, ME
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DennisG01

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Checking the anti-ventilation plate is a good idea - roughly speaking, 1" higher than the keel for every 1' of setback.

What's the max RPM for your motor?

You'd be amazed at what some slime and minimal growth can do - there's a good chance that's the issue right there.

Has your mechanic done a compression test? It's not the be -all-end-all of things, but it's a good idea.

Trim tabs are wonderful - but not having them is not the issue. Adding trim tabs is a completely different thing.

I doubt there's anything wrong with your current prop and the 17" pitch sounds about right. You would notice if there was damage to the prop and if you had a slipping hub then the engine would have over revved.
 

MooseheadDoc

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Checking the anti-ventilation plate is a good idea - roughly speaking, 1" higher than the keel for every 1' of setback.

What's the max RPM for your motor?

You'd be amazed at what some slime and minimal growth can do - there's a good chance that's the issue right there.

Has your mechanic done a compression test? It's not the be -all-end-all of things, but it's a good idea.

Trim tabs are wonderful - but not having them is not the issue. Adding trim tabs is a completely different thing.

I doubt there's anything wrong with your current prop and the 17" pitch sounds about right. You would notice if there was damage to the prop and if you had a slipping hub then the engine would have over revved.
Thanks for the reply, Dennis. Nice to see another post from Maine. I split my time between Moosehead and Deer Isle, but the Grady will stay on Moosehead.

Max rpm range per the Johnson manual is 4500-5500. I feel like I’d like to be able to see 5000, and cruise at 4-4200.

I’m really hoping the bottom is a big issue. If I can find someone to do it I would spend the money to remove all paint and wax the hull.

I know I blew-off the engine in the original post. But on that front- just prior to purchase all cylinders tested good compression, new VRO pump/power pack/plugs/wires, new tstat, optical sensor and carbs rebuilt with new float bowls. Now, none of that means anything to me. I do know the engine starts easy (even for a sea trial on a 40 degree Maine morning), idles smoothly, never stalls, and accelerates/slows without a sputter.

She just got hauled today, so I’m going to see if I can go check it out in the rack and have a better look at the bottom and the cavitation plate.

cheers... JB
 

seasick

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Without all the factors, it is hard to say what's going on but my initial feeling is that the prop diameter is too small.
 
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MooseheadDoc

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I've been off the boards for a while. Now that the boat is back in the water I'm trying to tie-off old posts. When I go down the rabbit hole of reading old threads it's nice when they have happy endings.

This Spring I'm getting much better performance- probably due mostly to better engine performance, and in part to bottom care.

My mechanic has the old Johnson running on all 6 cylinders. The bottom was scraped and painted. I'm getting 5200-5300rpm at WOT for 32-34mph by gps. She gets up on plane pretty well.

For now I'm going to declare victory and just run it. Later in the season I may try out some different props and see if there is a noticeable difference.
 

Hookup1

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Get your numbers together and call the guys at www.propgods.com . Even if you don't buy anything from them you will get a recommendation from well...Prop Gods! Seriously these guy know their stuff. Sometimes the smartest thing you know about your problem is what you know you don't know.

ProGods spec'ed mine last year and I could not have been happier with the results.
 
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seasick

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So are you saying that before it wasn't running on all six?