Pontoon Boat & Deck Boat Forum

Need advice for general setup of motor height vs. prop. I have got alot of good info from this site that I am trying to apply to a new to me pontoon boat. This is my second toon with the first being an old fishing barge. This is a 2001 20' Sweetwater with a 1991 mercury 90hp. The boat runs great and I havent had any engine problems so far, knock on wood, but I am sure that I am not running up to my full potential as far as set up goes as I am having a ton of prop slip. My original prop was a 13-1/8x16P stainless prop that would only let me hit 4800rpm at WOT at 20mph, thats too much prop with alot of slip. I read many posts on here as well as other sources online and a 14x13P prop seemed like a pretty safe bet for my boat so I went ahead and picked one up. Got to run the boat this morning in good conditions with a medium load on it and needless to say I was not impressed. My rpm's were just right hitting the 5500 rpm limit at WOT but I was only getting 16-17mph on the GPS. I am getting about 36-37% slip which interestingly enough is exactly what I was getting on the 16p prop. My cavitation plate is about 1.5" below the keel of the transom and while I have more holes I could drop the motor I think I could only drop one more before I hit a plate at the base of the transom. I have attached a video link for your enjoyment, haha, to see if anything out of the ordinary jumps out at your. It has a couple of pics and a video of the motor running wide open that I took this morning. Any advice would be appreciated as I know I can get more out of this as my old fishing barge was a longer boat with the same exact motor and I could hit 25mph on it.

Thanks again!

Video link address: https://youtu.be/AdmBOUXzroM

I think if anything you might have the engine 2 low as I didnot see the cavitation plate.

How does the nose ride. To much weight up front will slow the boat down. Try to get some side picks of the toons as you are going at different speeds.

I didn't get any pics over the weekend but since that video I raised the motor as high as I could without drilling new holes which was a lot of fun doing on the boat lift! The transom splash is now gone so that's good but I still can't see the cavitation plate, I can see the small plate above the cavitation plate clearly though so I'm not far off. I feel like I have a little more bite on the bottom end now but it didn't really make any difference in the top end. Since I have the motor height improved I figured I would try my 16P stainless prop along with the 13P turning point aluminum to get a better base line. Regardless which prop I choose I hit a wall at about 18-19mph with the 16P running at 4800rpm wot and the 13P running at closer to 5700rpm this equates to around 35% prop slip for either prop which I feel is just to high. These runs were with a slightly below average load. I have been emailing turning point and he told me the 15P would do the job, unfortunately I didn't know about the exchange when I fiat got the prop. I am hesitant that the 15P will drop my slip as much as I would like but I am hoping. I guess I will try the 15P 3 blade and if that doesn't do it return it for the four blade. Any other ideas? Thanks!

On another note I left the 13P on and had a pretty heavy load on Sunday. Basically no difference in too end still around 19mph and knocked my wot rpms down to 5600.

Jake_Daddy wrote:On another note I left the 13P on and had a pretty heavy load on Sunday. Basically no difference in too end still around 19mph and knocked my wot rpms down to 5600.

Sadly, unless you go to a bigger engine you're going to keep hitting that speed "wall". Its the nature of our beasts. My Cruising speed is at 4000 RPMs - my WOT is 1000 RPMs more for an extra 4mph... is it worth the extra gas and wear?

I would say stepping to the 15p from the 13p is going to get your speed up, but your WOT RPMs are probably going to be closer to where you are with the 16p (assuming your diameter is going to be 14 and your 16p was a 13.125). More prop to turn, lower pitch - could be a wash. People get too hooked on the pitch, and forget the diameter. On a pontoon you want a bit more diameter than a V-Hull so you can to grab as much water as possible per turn. The original 13.125x16 was faster because of the increased pitch. But not by much because it was slipping because it was too small My guess would be the original owner subscribed to what they saw on regular boat sites and just kept moving the pitch around to get the RPMs in the range, but never considered the diameter.

As you're set up now, the 13 isn't enough pitch because you're exceeding your RPM max of 5500 even under heavy load. But, that may be because you're engine is too high now. The cav plate being visible while underway isn't accurate for pontoons with an equal length and angled motor pod (which most older pontoons have) so you may have moved the engine up too high. If your engine is bracketed back a foot or so behind the pontoons like with newer pontoons, then yes. You should be looking for the cav plate. But your engine should be mounted so that while your engine and boat are level (a bubble level works fine for this) when you place a straight edge along the bottom of your motor pod it should come down and touch the cav plate. Think about the water being pushed under the motor pod and it'll make sense.

Leave the 13p on there. Drop the engine one hole. That should bring your RPMS down about 2-300. And you may be happy with it. You're not going to be a speedster either way. If you're not happy with its performance,then you could go into a 4 blade but stay at 13 pitch for more "bite" and responsiveness. But a 4 blade is going to bring your speed down even more.

this may help explain the engine mount height

https://patentimages.storage.googleapis ... 9331-2.png

Probably not exactly what you have - but regardless you get the idea. Look at the bottom of the motor pod - it's angled. So if you have the engine set up so high that you can almost see the cav plate, you may be too high. the angle of the motor pod may be pushing the water too low and keeping the prop from getting enough bite.

Put a straight edge on the bottom of the motor pod (towards the rear) and it'll show you (at least to some degree) where the water is going as you're moving along.

JerEazy, I would agree with your assessment on the motor pod if mine was angled. I don't have a pic but my pod is flat underneath at least not a noticeable pitch to it. Add to that I tried some pretty tight turns to see if I would completely blow out and that never happend so I think I am getting pretty clean water.

I don't think this would be driving me so crazy if my old party barge with the exact same motor, that didn't run near as well, didn't do closer to 25mph. Also I just feel like the slip around 35% is just too high, I don't expect to get close to 10% but I would think around 20% would be doable.

Does anyone with a 20' dual toon boat get over 20mph with a 90hp? I feel like I read numbers closer to what I am shooting for all the time!

yes, i have a 20 foot 2 toon boat with a 90hp evinrude and i average 25 mph (sometimes less, sometimes more depending on load) i am running a 14*13 solas. but the gear ratio between a evinrude and mercury are different the boats around here running 90hp Mercuries are running 15p props, i think the gear differences is what makes them able to turn the higher pitches.

Jake_Daddy wrote:JerEazy, I would agree with your assessment on the motor pod if mine was angled. I don't have a pic but my pod is flat underneath at least not a noticeable pitch to it. Add to that I tried some pretty tight turns to see if I would completely blow out and that never happend so I think I am getting pretty clean water.

I don't think this would be driving me so crazy if my old party barge with the exact same motor, that didn't run near as well, didn't do closer to 25mph. Also I just feel like the slip around 35% is just too high, I don't expect to get close to 10% but I would think around 20% would be doable.

Does anyone with a 20' dual toon boat get over 20mph with a 90hp? I feel like I read numbers closer to what I am shooting for all the time!

Bah - sorry then.

The other poster is right - I've got a Johnson 90 on a 21 dual toon spinning a 13.75x13 and doing about 25 at WOT. But, the Johnson 2-strokes run a 2.00 gear ratio and the Mercs push a 2.3... Not much, but the difference between 4th gear and overdrive? Maybe.

If it's not going to cost you too much, then I think you'll be happiest with the 15 pitch speed wise. But a 4 blade 13p will get your RPMs right, lower your slip, and give you better lower speed control.

Agreed that the gear ratios are not the same but my lower gearing should swing a slightly larger prop just at a lower RPM putting me right in line with you guys. If I can get a 15P and achieve a 20-25% slip and drop me about 400rpm I will be right at the 24-25mph mark which is right where I would like to be. If I cant get the slip lower than 25% I will swap for the four blade for better bite, we have a lot of Manatee slow zones and no wake zones in Merritt Island and the better lower speed efficiency would be nice!

Jake_Daddy wrote:Agreed that the gear ratios are not the same but my lower gearing should swing a slightly larger prop just at a lower RPM putting me right in line with you guys. If I can get a 15P and achieve a 20-25% slip and drop me about 400rpm I will be right at the 24-25mph mark which is right where I would like to be. If I cant get the slip lower than 25% I will swap for the four blade for better bite, we have a lot of Manatee slow zones and no wake zones in Merritt Island and the better lower speed efficiency would be nice!

google a prop-shop near by and make a call! they may have a few you can just test out.

Yea when I was emailing Turning Point they said will swap like new props for $25. I wish I would have known that when I first got my 13P a couple months ago before I got a couple scratchs on it! I can guarantee when I get the 15P I will do a test run and make a quick decision on whether to keep it or swap it.

Ok I can finally add to this thread for anyone who may someday stumble across it! I picked up a turning point 4 blade 13.5x15P and took it out for a spin this morning. My top end did not change at all from the 13P I had on there, I'm still hitting the wall at 20mph. It did drop me perfect in my RPM zone around 5300rpm at WOT with my normal load so that was a good thing. I really liked the prop in the 3000-4000rpm range and the boat was noticeably quieter with this prop over the 13p three blade im not sure how to explain that but it was a nice bonus.

The interesting thing is my slip numbers went up from ~35% to around 38% at WOT which is unexpected going to the four blade. I don't think that was the case throughout the mid and lower rpms as it noticeably had more bite. Also I ran over some big wake from some large cruisers to see if I could break it loose and it never did.

Reading another thread someone had skinned a 2001 aqua patio and gained 3mph top end and I am assuming it was probably built similar to my Sweetwater. I think that's going to be my next move, even in pretty nice conditions my floor still gets a water spot about 5' back from the bow which tells me those cross members are just getting pelted with spray and maybe that's what's holding me back.

If anyone has any other suggestions let me know!