Motor height and prop setup...

Jakedaddy

Recruit
Joined
Jan 22, 2013
Messages
4
I have a new to me 2001 Sweetwater 20' with a 91' Mercury 90hp. I have had it for a couple of months and the motor runs great, knock on wood, but would only hit 4800rpm at WOT @ 20mph with a 13-1/8x16P prop. I assumed that the low RPM was the over pitched prop and the high slip, +35%, was the small diameter. Last weekend I installed a turning point 14x13P prop and was able to hit close to 5500rpm which is great but only got to 17-18mph which still equates to +35% prop slip. I have extremely high slip at any engine speed and didn't see much difference between the two props outside of sounding different. I now believe my motor may be too low in the water. At the moment the cavitation plate is about 1.5-2" lower than the bottom of the transom plate, I could possibly lower it one more hole if that was the issue. I have liked a YouTube video that has some motor pics as well as video showing the boat running at wot, it's really turbulent back there! If anyone has a suggestion on making motor adjustments or anything else I would appreciate it. Thanks.
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
You have a pontoon -- not a planing hull so slip is going to be greater. Switching brands of props can have performance differences as well. Are you checking speed with the dash speedometer or by GPS. Even your smart phone can do GPS speed which is far more accurate. And are you certain the tach is reading correctly. Both of those must be verified or you are chasing your tail (so to speak). A 4-blade pontoon-specific prop will work better than a generic prop designed for planing hulls. Besides, those big elephant ear blades help navigating in reverse as well and help provide more "push".
 

HotTommy

Lieutenant Junior Grade
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Mar 15, 2013
Messages
1,025
Jakedaddy,
Another recent thread here was from a man with a 20' pontoon boat and 90 HP outboard. He too was chasing RPM with prop changes. I ran his (your) specs through a propeller selector website and it suggested ...... a 13-13.75" diameter prop with 16" pitch. That sounds like what you started with. .... I doubt you will get over 21 MPH no matter what you do with props or motor height. So my suggestion is to put the old prop back on if it is in good shape, and save the new one as a spare. Then enjoy you boat. .... BTW, if you do use GPS for speed comparisons, keep in mind that weight, wind and currents do have an effect, especially on something the size of a pontoon boat. You could easily see a boat getting 21 MPH with a 10 MPH tailwind, and 17 MPH going back into that same wind.
 

MH Hawker

Vice Admiral
Joined
Jul 13, 2011
Messages
5,516
when i was setting up my new motor, i started out by dropping the motor as low as i could, that put it in clean water for the best prop bite, then after i changed props around for max rpm then i raised it till the rpm started to jump up then dropped it down a set of holes
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Seeing vid, that OB sits too low on transom, there's water splashes against transom area, dial OB height for water flow to skim right under upper small water def plate while trimmed exactly same as vid. Once there can go for a prop maximization to pull wot revs towards max wot range as currently loaded. With toon deck weight evenly balanced. OB and prop will work much better.

Happy Boating
 

Sea Rider

Supreme Mariner
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Sep 20, 2008
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12,345
MH,

Trimmed same as vid with well distributed weight on deck, raise OB accordingly for (WF) water flow to skim right under small upper water def plate (bluish line) at fast displacement speed. that's the sweet OB/transom height for that 90.

Merc 90 HP.JPG

Right now water flow is banging at speed at X1 area producing back splashes towards transom area, this cond will slow down combo a bit. Once OB is raised spot on will achieve a near flat middle wake out OB. If raising OB too much towards X2 area will achieve excessive prop aereation, too poor water level for prop to grip well. Set OB height as suggested and report back.

Happy Boating
 

Jakedaddy

Recruit
Joined
Jan 22, 2013
Messages
4
First of all thanks for the replies. I was on vacation and didn't check this for a week. To update my progression on this I did raise my motor up as high as I could go, two holes, without putting more holes in the transom the large cavitation plate is now about 1/2" below the keel. I was only able to take a quick 5 minute test ride in some pretty rough conditions but there was quite the noticeable difference already. My hole shot felt way better and i could not go wide open because I was running over the suggested WOT, I hit 6000 rpm and dialed it back to the 5500rpm. Unfortunately I left my phone at the dock so I didn't have my GPS speedo which is what I have been using. I am sure I will get out this weekend unless the weather is terrible and I will update further.

To address some of the comments. I know I am not going to be hitting slip numbers in the 10-15% range but I would like to think close to 20% is achievable and I have to think I can improve as my old toon a 24' fishing barge would hit 25mph with the identical motor, can't remember what prop was on it which is too bad!

Also the transom wash is gone now and everything is much less turbulent. I am going to update my video to show how it is now for others to reference, I will post a link once I get a chance to take another clip of it running.

When I run the boat this weekend I am going to do two more experiments. One is swap the props and take down some numbers to see what I get and another is going to be taking off the big anti cavitation wing on that thing. I have read pros and cons about adding those big wings and it seems like it may be a bandaid for an improperly set up motor and completely not needed.

Again thanks for the guidance and if y'all have any other suggestions please let me know!!
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
In reality any given physical distance from cav plate to keel doesn't say much, what need to do to get the max out of any OB is match lower leg to water flow as suggested. As you have experienced when rising OB toon has performed much better than before, water splashing drag on lower tail once released has made the OB to rev way more. Let's see on nex vid how much water splashing at back transom has been lessened. Ideal is to have 0. Check on flat calm no wind water cond if at close turns prop ventilates/aereates ?

Is it possible to plane that toon or just good for fast displacement speed. 6000 revs was achieved while lightly, medium loaded. That OB should have a rev limiter, not an issue testing at full wot, it's just for a very short time, probably rev limiter will kick in before going full hammer down. Ideal is to be propped right for OB to rev around max wot revs as usually loaded.

With respect to lower leg doel fins, if don't ride parallel to water level when toon is at fast displacement or plane speed will achieve water drag on them, could slow toon a bit, personally would remove them, are intended for planing speeds rather than fast displacement ones.

Happy Boating
 

ahicks

Captain
Joined
Sep 16, 2013
Messages
3,957
Just keep in mind that most pontoon boats need to turn at some point. If the engine is installed too high, any turn that might be fun on a tube will cause the prop to cavitate. IMHO, a hard turn to each side should be part of any testing done regarding motor height.....
 
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Jakedaddy

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Joined
Jan 22, 2013
Messages
4
Ok well I was out a lot last weekend and tested both props again to have a bit more data still with +35% slip at 18-19mph and that didn't seem to change with the load I had on it either maybe 100rpm change going from two adults to four adults and four kids.

To comment on Sea Riders question...I don't know if you call it planing or fast displacement but at around 4000rpm it goes from plowing with the bow up to leveling out and picking up speed. Feels like planing to me. I wouldn't be so frustrated with this if my older larger boat didn't run faster with the identical motor, that never ran right!!

Anyway the guys at turning point told me to try their 15P prop so I guess I will try the 3 blade first then return it for the four blade if it doesn't work any better.

I will report back later but in the mean time if anyone has any suggestions pass them along!

Thanks again.
 

Silver Eagle

Master Chief Petty Officer
Joined
Mar 16, 2010
Messages
852
Check your motor's trim while moving. I had a floating level on my toon mounted on the side rail You'll be surprised how much difference there is by just trimming your motor.Your speed will change ,keep an eye on your speed ,it will increase or decrease.
 
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