Motor upgrade

My family just bought a 199?'s 16ft Sweetwater pontoon. It has a 35hp Force on it. Loaded (two adults, one child, dog, cooler, etc.) GPS says we are doing about 11 mph wot at about 4300 rpm. We would like to be able to do some tubing with this thing, therefor I am looking into a way to gain some much needed speed. The obvious is a bigger motor. It is rated for a 50hp max. Since we are new to the world of pontoons I am looking for any input to help us out, thanks.

Re: Motor upgrade

You need to give us more info, what is the WOT rpm's...what size prop, it's possible that you may only need a prop change or motor adjustment.

Re: Motor upgrade

I think your current motor is either over propped or not running right. With that said you will definitely want the 50hp if you want to do any tubing. It won't be a power demon but will pull people through the water, especially the kids. With a good running, properly set up 50 you will probably get up into the low 20's

Re: Motor upgrade

Also, for an inexpensive speed increase, skin the bottom of the boat platform, if it is not already done. It is the easiest and cheapest way to get extra mph out of a pontoon. The skinning will keep water from splashing your cross members, and acting like a break.

Re: Motor upgrade

I don't really think any water is splashing anywhere at 11 mph.

Re: Motor upgrade

^^^^ All 4 responses are right on track. There may be 1-3 mph hidden in your rig as it is but the WOT specs on the motor and the gear box ratio are needed along with the prop pitch for the calculations. From there, it can be estimated how much improvement can be made before you change motors. At 4,300 rpm there's something wrong for sure and you're probably not getting 35 HP as it is. The motor is either weak on HP or it's over propped or maybe some of both. The point is you may have a few mph hidden in your rig and when you change the motor to a 50 HP then you know you're working from a base of 12-14 mph instead of 11. If you can get the specs mentioned then the calculations can be run to see where you're at.

Yes, not much happening at 8-10 mph underneath but once you start picking up speed you will hit some splash resistance under the floor especially in the turns pulling tubers. As mentioned, the under skinning is one of the biggest bangs for the buck there is on pontoon boats. There are plenty of threads on this so just do a search on this site for "under skinning".

The bottom line is, with a "strong" 50 HP motor, a premium prop selected for your rig and under skinning you may see 20 mph or so. If you can get it to turn then you can give the junior tubers a decent ride but as mentioned, it won't be a power house.

I would inspect the transom and make sure its solid so the additional HP won't cause a problem. I've seen pontoon transoms that you wouldn’t want to put a 9.9 on and then some that look like they are built for the rated HP plus. Just shake it around and look at the way it’s secured. You can buy heavy duty transoms if yours in marginal.

Re: Motor upgrade

An under 20' pontoon boat is very rare in many places, as their toons are small and they're not capable of heavy loads.

Do yourself a favor and try to sell your boat on the retail market. Then purchase a longer boat that's more seaworthy and capable of handling a motor that suits your family's needs.

When you get to 20' boats, you can purchase a 22' boat for just slightly more. But if I was into water sports, I'd be looking for a 20' with 90 hp or a 22' with 115 hp or (even better) a 24' tritoon with 150 hp.

Underskinning helps, but the best option on a pontoon/tritoon is a set of lifting strakes on the toons. They lift your nosecones out of the water for better fuel economy and another 5 mph.

I've got a Starcraft 24' pontoon with a Yamaha 115 hp 2 stroke. It has plenty of power, but handles like a pig. I purchased a new Bennington 24' SSLX tritoon last year with a Yamaha F150 four stroke. It's the cat's meow @ 40 mph top speed. It'll do anything I ask it to do--including make tight turns for pulling tubers.

Re: Motor upgrade

lakegeorge said:

You need to give us more info, what is the WOT rpm's...what size prop, it's possible that you may only need a prop change or motor adjustment.

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rpm's wot are in the 4200-4400 range, again that was loaded. haven't had it out with just myself in it on a calm day. As for the prop size, good question. This is a 1991 Force and I'm guessing it still has the original prop on it based on how it looks. Is there an easy way to tell prop size and/or pitch? I know it is a 3 blade, but that is about it. Also the guy I bought the pontoon from overheated the motor in weeds. I went through and cleaned it and replaced as many parts as I could that needed to be, but was limited due to the age. Not knowing much about boat motors it seems to be running alright; idols smooth, starts right up, doesn't drink fuel by the gallons (I know, all of this is pointing to a new motor:facepalm. I know it will never be a speed boat, just didn't know what to expect when I put a bigger one on. Thanks to all who have replied!!

Re: Motor upgrade

Is your motor rated for 4200-4400 rpm's or is that what you get at WOT? What is the motor rated for? You need to take it out by yourself to find out what the difference is when loaded.
The prop will have numbers on it and you may be able to read them without taking it off. Let us know.

Re: Motor upgrade

you need to set the motor up for your average load, I would think 2 adults 1 child and 1 dog would be what you were probably shooting for. I don't know what your WOT should be but I suspect it is between 5-5500 rpm. That motor may be fine, but check your compression to help verify, shoot for hopefully over 100 psi and within 10% between cylinders. If that turns up good then figure out you props pitch by cross referencing the number that is hopefully on there and dropping pitch. For example if you current have a 15 pitch you will probably want to drop to a 11.

Re: Motor upgrade

crb478 said:

you need to set the motor up for your average load, I would think 2 adults 1 child and 1 dog would be what you were probably shooting for. I don't know what your WOT should be but I suspect it is between 5-5500 rpm. That motor may be fine, but check your compression to help verify, shoot for hopefully over 100 psi and within 10% between cylinders. If that turns up good then figure out you props pitch by cross referencing the number that is hopefully on there and dropping pitch. For example if you current have a 15 pitch you will probably want to drop to a 11.

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I had to replace the head gasket so I checked compression and both cylinders were above 100. the top was at 110sh and the bottom at 107 I think. Where would I find what RPM's the motor is rated for? Is it on the motor somewhere, or do I need to do an online search? Next, is it worth spending money on a new prop (vs putting that towards a new motor) if it is only going to give me 1-3 more mph or am I looking at a much bigger increase in speed with the right prop?

Re: Motor upgrade

The RPM range depends on the year of your engine. Below are the RPM ranges. It is important to prop the engine to run in it's designed range in order to prevent the engine from lugging (or working too hard) under normal operating conditions. (Think of it as a manual transmission car pulling a steep hill in 4th or 5th gear) It is generally best to prop toward the higher end of the RPM range with a light load on the boat. That way when you have a load on, it will still stay within the recommended range.


1986-1987 4500-5500
1988 4500-5500
1989 4250-5250
1990 5000-5250
1991 5000-5250

Re: Motor upgrade

I can’t see it getting where you want to be with the 35 hp. If you have the motor where it runs good with the repairs as you've mentioned, I would try to sell it and trade up but you'll still have to go through the prop game with the new motor. I just propped a bass boat and it took 5 or 6 trips to the river with different props and motor height adjustments. When you get close with stock props then you can either stick with one of those or order a nice one.

I would echo a concern about the pontoon size and the general structure of the transom. If you would post the total length of the pontoons and the diameter if they are round pontoons or the diameter and height if they are "U" tube pontoons along with the weight rating on the boat ID tag, it would let us know what kind of toons are under there. I've got a short pontoon boat without a motor on it (not sure if it's a 16 or 18) floating in a small lake that we use as a dock and I wouldn't hesitate about putting a 50 HP (or so) on it but I might reinforce the transom a bit because of the way it’s made.

Based on the charts I’ve seen, my experience and other posts, you should get in the low 20’s (21-23) mph from a strong running and properly propped 50 hp on that 16 foot boat. When you jump to a 20 footer it would take a 75 hp to do the same thing and a 90 will get into the skiing range. When you go to the 22 – 24 footers you need to be in the 115 – 125 hp range to tube well and still just fair for skiing. Then you get to the tri-toon with 150+ that will do about anything you want. I just changed from a 130ish HP on a 24 foot tri-toon to a 150 HP and it made all the difference in skiing. I could ski behind the smaller motor but it is a struggle for me to ski in the upper 20’s mph.

When you are shopping for motors there is a break somewhere in the mid 80’s where they started rating motors at the prop instead of the engine. The older motors are several HP short compared to the newer ones. If you get an older 50 HP it’s really closer to a 40HP.

With that short of a boat assuming it will be ok with the 50 hp, you could get to the point of tubing small to medium size kids if the boat is loaded reasonably. My last 10-15 years has been pulling teenagers to young adults on tubes and some of us old guys that still like to ski and I don’t think this boat with a 50 hp on it would do that. If you’ve got a few years until the teenage years (the big boys), then you can probably find a 50 HP and get by with it a few years and have a lot of fun.

Re: Motor upgrade

Thank you to all who are replying. It is really nice to have a site where so many people are willing to help!
With that being said...had the boat out today with just myself. @ WOT, it was running about 4700 rpm and 11-12 mph on the gps. It was a calm day with almost flat water so good conditions to do some testing. I looked at the prop and cannot find any markings at all. Still looking at re-powering with a 50 (looked at the transom and it's SOLID), but now I am curious about tuning the 35 up by re-propping now just b/c I like a challenge...must be a guy thing, lol. Since I don't know what size prop I have should I start small and work my up, or start big and work my way down in prop size?

Re: Motor upgrade

Your prop size or a least a part number should be on the prop hub where the thrust washer goes up against it. You will need to remove it to see it. it will be stamped and sometimes is faint and hard to read but should be there.

Re: Motor upgrade

Most but not all props will be marked, but often you have to take it off to find the numbers. Failing that a good prop shop can measure it and verify the pitch.

Re: Motor upgrade

your speed is off by quite a bit, My 20' came with a wore out 30hp prop rated 28SPL rude' and would run 15-16mph GPS with just me. Your 35 Force is also roughly a 30hp prop rated engine and your only seeing 11-12mph so there is something wrong somewhere in the setup OR power department. Not sure how much your looking to spend but I've seen older 50hp 4 strokes down in the 2500$ range lately and some off them were even EFI engines.