Slow pontoon boat 50hp merc, installed a 90hp merc, no change in gps speed

[dapoppa]

Running a 2005 Suntracker Regency 22' with 26" pontoons, necessary on our lake with all the idiots trying to run us over or swamp us. Came w a 2005 50hp merc 2 stroke. Ran wot at 5400 and 21-22 mph gps. I was not happy with that speed so bought a used 2007 90hp merc 2 stroke we took off of a low profile fishing boat doing 47mph. 13"x17p prop, just not fast at all on the pontoon. Put on a 14x17p and not much help at all. Called a bunch of dealers and got advice off the net, and the consensus was that a 14x15p was the perfect prop. I had the 14x17 prop reduced to a 14x15p and it was not great either, only did 4600rpm, so it got damaged in a battle between my prop and a railroad tie during a raging thunderstorm.

After that I just bought a power tech 13.75x15p new, the wot went from 4600 to 5400, right where it should be and the same as the 50hp. The speed with the 90 now is as follows:
700 rpm idle 2mph gps
1000 rpm 3 mph
2000 rpm 6 mph
3000 rpm 9 mph
3500 rpm 12 mph
4000 rpm 15 mph
4500 rpm 18 mph
5000 rpm 19 mph
5400 rpm 20 mph, 21 mph w a stiff tail wind, no load and a dolphin pulling me.

The law of diminishing returns seems to kick in about 4500 rpm and it just drags along with no great increase in speed. Now I did not expect the near doubling of hp to nearly double my speed, but I did expect SOME increase for heaven's sake. I used to run all day on a half tank of gas, now it takes nearly a full tank. Last time out I got 2.2 mpg ! The hole shot is twice as fast as the 50hp, I generally run it at 3500 to conserve gas. It is trimmed correctly with the tubes 1/3 or more out of the water. The steering does not pull and I am able to drive hands off, so it is trimmed in my experience and opinion. Both motors are long shaft.

The cavitation plate is about 10 to 12" below the water line, but the 50hp was about the same. We checked 5 dealers in person looking at the new and used boats on the lots to see where the motor was mounted. We installed ours in the same holes. The motor is mounted a foot or more off the rear on a separate motor mount, not on the aft part of the boat. I am at wits end. I cannot afford 24 diff props to try out since I'm retired and its in a slip, so installing a new prop is a royal pain. Any thoughts and advice would be appreciated. Thanks. Ed.

 

[rickdb1boat]

Re: Slow pontoon boat 50hp merc, installed a 90hp merc, no change in gps speed

The cavitation plate is about 10 to 12" below the water line,

You have found your problem. Is the motor you have on there now a long shaft (20") or an extra long shaft (25")? 10 to 12 inches below the waterline is very deep..

In this picture, with my 90 HP 4 stroke, you can see the shadow of the ventilation (cavitation) plate just below the water by about 4 inches. My pontoon logs are 25" round ones..

 

[rickdb1boat]

Re: Slow pontoon boat 50hp merc, installed a 90hp merc, no change in gps speed

Also, I run a 13.75 X 15" pitch Mercury Black Max 3 blade prop (Aluminum) and I get 28 MPH with just myself onboard.

 

[MaPaHa]

Re: Slow pontoon boat 50hp merc, installed a 90hp merc, no change in gps speed

Unless I mis-calculated or I'm using the wrong gearbox ratio, you have almost a 40% slippage. I looked up your motor and it seems that it has a 2.33 gearbox ratio based on what I saw. If so, your 15 pitch prop at 5,400 rpm gives a true speed of 33 mph before slippage. At 20.5 mph actual speed that’s a 38% slippage factor. I checked it at your 4,000 rpm data and it's about the same.

This indicates to me there is too much drag coming from somewhere (most likely) or the prop is ventilating (less likely). For sure check to see if it's a long shaft (20") or X-long shaft (25"). Also, what condition are the toons in as far as dents and how clean are they? Is there a chance there is water inside them? It's amazing what dirty toons will do to mph.

When you find the source of the drag and correct it, you should pick up some speed.

 

[dapoppa]

Re: Slow pontoon boat 50hp merc, installed a 90hp merc, no change in gps speed

I agree w you. I had no idea it was so high a percentage. Both motors were elpto, long shaft. 2.33 is correct according to records. As far as drag, I keep it in a slip, no boat lift, so the toons could be full of algae I would assume. I have to trailer it for 5 months due to weather here in MO, so we did scrap off as much of the algae as we could while on land. We waited until it was dry and it came off nicely, but where the boards were, no luck of course, about 2/3 of the algae came off. But it grows back. of course. No dents under water. I checked for toon water on trailer, none indicated. And there's the log deflector which HAS to cause some drag. Can dirty toons cause 8mph loss?

 

[dapoppa]

Re: Slow pontoon boat 50hp merc, installed a 90hp merc, no change in gps speed

I would be ecstatic w 28mph. I came from a life with a runabout that would do 37-38mph on a calm day. It was an inboard 4 cylinder chevy mercruiser. These are my first 2 outboard mercs. My boating buddy has a 21' '97 suntracker signature, a lighter boat with 24" toons, and he gets a bit more than me with the same motor and prop. Maybe 2 mph more. Again, perplexing for sure. His motor is mounted the same and the cav plate is deep also.

The playpen is 22' but there is a sunning deck behind it with a changing room, battery and fuel tank right in front of the transom. So a lot of weight in the back. Most people like to be under the bimini, even more weight in the back. I will try to get them all up front and see if that helps, but that might cause more drag with more of the toons in the water. Where to go next???

 

[dapoppa]

Re: Slow pontoon boat 50hp merc, installed a 90hp merc, no change in gps speed

The motors are both elpto, so def long shaft. The depth seems way too much to me also and the amount of turbulence behind the prop seems excessive, I guess the boiling of the water and the movement of air instead of water. It just seems way too much to me. However, there has never been any pitting of the prop that I could see. I do not see the cav plate from the surface. The motor runs just fine and there is no hesitation and the idle is pretty smooth considering its a 2 stroke. I will have to double check if we are in the top hole on the mounting plate, but 2" would not make that much diff, I would think. It does sound like it is too low based on info in the books. Not sure if I can buy a diff mount or an extension to the one I have. I am 200 miles from the boat and will be going down mid Oct, and will double check the cav plate and the hole we used. It was the SAME hole as the factory used for the 50hp. I have seen cav plates that were mere inches from the waters' surface. Seems a logical solution, but one that is not easily remedied. Will inform.

Thanks for all the suggestions and information. Ed.

 

[Scott Danforth]

Re: Slow pontoon boat 50hp merc, installed a 90hp merc, no change in gps speed

I would try to raise the motor

 

[The Woo Woo Kid]

Re: Slow pontoon boat 50hp merc, installed a 90hp merc, no change in gps speed

Most people like to be under the bimini, even more weight in the back. I will try to get them all up front and see if that helps, but that might cause more drag with more of the toons in the water. Where to go next???

Put the top down. Try again.

 

[Silver Eagle]

Re: Slow pontoon boat 50hp merc, installed a 90hp merc, no change in gps speed

We have a 24.3 limited Ed StarCraft with a 90 hp two stroke 2007. I usually cruise around with just the two of us at 3500 rpm's and a speed of 16 mph. In the picture the motor looks like it's way to low in the water. I also cruise with the top open.

 

[dapoppa]

Re: Slow pontoon boat 50hp merc, installed a 90hp merc, no change in gps speed

Thanks again for all the help and suggs. I think we are zeroed in on the problem and perhaps may have the bull by the proverbial horns. Loved the post from woo, great sugg. I do think the motor is low and in a couple weeks will be able to prove or disprove the suggs. I did like the 50hp and the gas use was much less for sure. The 90 sucks it dry. Will inform in a couple weeks when we try it out. It looks like a jack plate is in my future. Think I can get some bamabucks for this project??? If it works, it would be great for my mental health, and my wallet. Surely somewhere in that 21000 pages there is a provision for boaters' health, one would think, eh. Thanks again, ed.

 

[MaPaHa]

Re: Slow pontoon boat 50hp merc, installed a 90hp merc, no change in gps speed

As a reference, my boat is tri-toon with lifting strakes on all three toons and the boat measures 20’ pin + 2’ swim deck with 23.5’toons at 23” dia. I had an old 77 model 140 hp Evinrude (about 115-120 in today’s prop rating) and with a metal top I was lucky to get 28 – 30 out of it. The top off does make a difference and clean toons do as well. It’s probably too cold already but one way to get them clean is to scrub them in the water with scotch- brite pads.

It’s hard to say about the motor height but if you can get someone to drive and sit in the back seat and watch where the water hits the motor while its running that would give you an idea of what’s right. I had to move my motor lower than you would on a typical boat because it came out of the water in the turns and when you trimmed it out also. Right now it’s set to where the cavitation plate is about an inch below the water line while running. So I would say raise it up until it starts showing signs of ventilating and drop it back down a bit. This is where the jack plate would make it easy to adjust and move the motor back to cleaner water.

28 mph would put you at a 15% slippage which would be pretty good. Are you getting any drag from water under the boat hitting the floor joist? If so, skinning the underside would help and there are several threads on that topic in the archives. If you can’t find enough drag to relieve, you’re at a point where you need more lift to take advantage of the larger motor. At one point before I added my center toon, I installed a Pontoon Water Glide and it did everything they said it would do. It got the boat out of the water and helped it turn much tighter. Lifting strakes could help as well but… any welder adding them has to know what they’re doing or they can mess your boat up. Both of these solutions are costly so the biggest bang for the buck is to find resistance you can eliminate including installing the under skinning.

I'm with you on the boaters mental health. Reminds me of one my favorite comments about health care: A well known political figure was ask what they thought about health care and the cost of it and he said: I think we should get off the couch, eat right, exercise and stay healthy and that would take a large burden off the health care system! Think I’d have to agree because I’ve always thought better maintenance contributes to fewer break downs.

 

[dapoppa]

Re: Slow pontoon boat 50hp merc, installed a 90hp merc, no change in gps speed

MaPaHa, thanks for the detailed message. My playpen is 22', then aft is the sunning deck/changingroom, /battery,/ fuel tank and dive deck and I would guess all that to be 2' approx. I think the toons stop right there and then the motor is offset about another foot. Then there's about a foot in front of the playpen, So I would imagine it to have about 25' toons, something else I will check in 12 days [yes i do count the days] . Also my toon is 8.5" wide instead of the normal 8', so that adds something I guess. Its heavy for sure. I don't like using a lift although I have one because getting into a 10' slip with 6" more width is a problem w my lift since the mounting bolts are right there to run into. The other reason i do not use it is I would be charged the yearly rate instead of the seasonal 7 month rate, a bit more of a hit on my meager budget.

We actually scrubbed the toons and the prop area last year. I noticed no discernible improvement, but then two old coots swimming around and scouring in the water proved to be less efficient than doing it dry on the trailer after the season with ice scrapers. My top is cloth, so not much weight there, but the wind drag is an issue for sure. I keep it up cause I have to due to health issues. I could take it up and down, but its taller than I can reach and a pain if no tall swabbies are on board. ;] It fits in the dock with 2" of clearance with the bimini, but I cannot raise or lower it in the dock due to the arch it makes.

Strikers, skinning, jack plates and water glides are all new terms for me. Again, came out of a runabout world. I have been gleaning some info from youtube and articles here and on the 'net. The jack plates are def an expense and the pneumatic ones are really $$$. I guess I will try a stationary one first and just move the motor manually. As far as health, when I got my first color tv in 1969, it had a remote and I thought to myself as I gleefully hit the buttons and changed channels and raise and lowered the volume while ascertaining that Spock was really green, I thought 'this is going to make tv very easy but its also going to make me fat' and boy was I right...

Thanks again for your info and suggs, I appreciate it. The season is winding down and much of this will have to wait until spring since my boat/trailer stays in the front yard of my little piece of paradise at the lake during the off season. It costs $80 to drive there and back, so winter trips are rare. Plus when I 'shut down' the cabin, the water and sewer lines are blown out and the water is turned off, so no living there then for 5 months. ed

 

[cc350]

Re: Slow pontoon boat 50hp merc, installed a 90hp merc, no change in gps speed

More weight with new motor means, boat sits lower in water causing more splash at transom. Look under the boat from the front while moving at different speeds ( someone driving boat/ yes it's not the safest thing to do) My guess would be to underskin pontoon to get the results your after.

 

[dapoppa]

Re: Slow pontoon boat 50hp merc, installed a 90hp merc, no change in gps speed

cc350, gonna test that theory out in 11 days and see if shifting some weight up front makes a diff. the new motor was only 101 lbs more than the 50hp, that's a half a person in my crowd, so if i can get the 3 guys who normally go along up front for a while and see if it helps. part of it is the weight i bring on board, but 10"? I dunno if it would make that much diff. I am almost convinced that they just throw on a motor mount and say, 'yay, that's close enough' and sell it. I am sure they sell jack plates also. the 50 performed the same way, crapping out after around 4000 rpms and adding little to the speed, while gulping the gas above that. I also measure the 10" on a trailer front the normal waterline on the toons, and its deep. Thanks for the sugg and thoughts. gonna see what we can figure out in 11 days and maybe visit my mech and see what he says. Thanks again to all, I am looking into the skinning thing next, find out what that's all about. did not have to do such things with my ol' runabout. sigh...

 

[dapoppa]

Re: Slow pontoon boat 50hp merc, installed a 90hp merc, no change in gps speed

Everyone: Thanks again for the help. I was on the lake yesterday and before we went out I placed 2 people in front and one in the back to level it out a bit. The guy in the back measured the depth of the cav plate and it was 10.5", well below what it should be. This confirmed my original post and the jack plate seems to be in my future. I have a motor mount that sticks out behind the transom about 15" and will have to investigate if a jack plate can be mounted to this 2" thick aluminum mount. I have been told that the reason for these long motor mounts is to move the center of gravity back and somehow improve the performance of the motor/boat. I guess that is true.

Since it is out so far already, I thought a 6" jack plate would be appropriate, no more, if it can be attached to the 15" mount. The longest vertical movement I found so far is 5" up, so it will have to be mounted high enough to get it up an additional 5" on the mount first and adjust the thing with a center bolt up or down with a socket wrench after loosening the side bolts, to find the 'sweet spot' in speed and gas use. At least that's the theory. Never been keen on theory ;]

I did check out pontoons today with those motor mounts and they were 18, 20 and 22" on some of them, so perhaps its a result of the length of the boat. And now they install the gas tank IN the motor mount which has to be a pain for the college kid that fills the gas tank. so they took away the crawling all over the aft to fill the oil tank in the motor with the new 4 cycle motors and replaced it with crawling all over the aft with a hose capable of pouring 1000 gallons of gas into the lake, go figure ;]

Any other suggs would be appreciated and thanks again for all the help. This is a whole new chapter in this 67 yr old sailor book of life. I thought outboards were easier than my inboard runabout, but I am beginning to doubt that thought. I never had to climb off the transom to put gas nor oil in the inboard. Any thoughts will be welcome. My mech will get a call monday re the install. Appreciate the help. Ed

 

[Scott Danforth]

Re: Slow pontoon boat 50hp merc, installed a 90hp merc, no change in gps speed

usually the length of the mount is so the cowl/powerhead of the outboard doesnt hit the railings on the pontoon when you tilt the motor up. The bigger the pontoon, the bigger the motor, the longer th mount to account for the motor.

Moving the CG back doesnt really help performance, it is just the nature of the beast on a pontoon (Move the motor out of the party area so there is more room for people and coolers)

The reason the gas tank is there is because it is usually wasted space and that is a nice convenient spot to put it. dont overthink it and assume any of the placements by the manufacturers were for performance. They are all for convenience of putting components after they have place things like the helm, seats, railings, etc.

 

[dapoppa]

Re: Slow pontoon boat 50hp merc, installed a 90hp merc, no change in gps speed

usually the length of the mount is so the cowl/powerhead of the outboard doesnt hit the railings on the pontoon when you tilt the motor up. The bigger the pontoon, the bigger the motor, the longer th mount to account for the motor.

Moving the CG back doesnt really help performance, it is just the nature of the beast on a pontoon (Move the motor out of the party area so there is more room for people and coolers)

The reason the gas tank is there is because it is usually wasted space and that is a nice convenient spot to put it. dont overthink it and assume any of the placements by the manufacturers were for performance. They are all for convenience of putting components after they have place things like the helm, seats, railings, etc.

All three thoughts make sense also. The 50hp had no problem missing the railing/and sunning deck, and there is no way the 90 will hit it either. The manufacturers sure don't seem to care about performance on the pontoons and I understand that, its not built for speed and I doubt they ever are unless someone rigs their own for racing or something. But I just would like a better bottom line on the gas use. The performance diff between the 50 and the 90 is nil and not worth what I paid for the larger motor.

I apparently paid just to get a better hole shot which is the case, but I hope that raising the motor will get better fuel usage. If i had the chance, I would put the 50 back on and save the gas money. But experimenting is part of being a captain/owner I suppose. I saw a pontoon w several motors doing 53 mph with only the aft third of the toons in the water. I could not see if there were jack plates or not but assume there were some. Thanks for your suggs and thoughts. The life of a captain is never easy [nor cheap] so hurry up Spring!!!

 

[Scott Danforth]

Re: Slow pontoon boat 50hp merc, installed a 90hp merc, no change in gps speed

World's Fastest Pontoon Boat - Brad Rowland's South Bay 925CR - YouTube

106mph

even a brick will fly if you put enough power behind it. the key to speed in a boat is to go from flotation to planing, or you can only move as fast as your hull is designed to go (bow wake theory)

If your desired performance is fuel economy, then enjoy the pontoon at just above idle, relax and grab a beer. If you want to go fast and get good fuel economy, then you need a different type of boat.

However raising your motor will help get you a bit of both and optimize what you have.

 

[dapoppa]

Re: Slow pontoon boat 50hp merc, installed a 90hp merc, no change in gps speed

Hello all, got to talk to the chief head big shot at the bass pro place's service shop. Getting him to pick up the phone is almost impossible, so I milked it for all I could. He indicated that the motor is in the correct hole [after the swap out from 50 to 90] so that is ok except the acp is far too low in the water. I know for a fact that had we installed it higher, the cables would have been to short and the steering would have been hard and the life of the cables would have been shortened. A friend tried it on his fishing boat and the steering was so hard he had to actually lower the motor back down. He was doing 72mph and lost 8mph when he lowered the boat. his steering was almost rigid and he just got tired of it. I think he had a 115 or 125 merc if there is such a thing. So we know it works on a low profile fishing boat.

Perhaps I am beating a dead horse as some feel, but he felt that instead of my 13.75x15p prop, he felt a similar size with a 13p prop would help some. He indicated that my wot was 600rpm too low and that the 13p would get me closer to the motor's high range. I do know that when I went from a 17p to a 15p, I picked up 500rpm. I need a spare anyway, so for $100 I could give it a try and get a 13.75x13p. I still don't understand getting almost exactly the same wot speed with a 50hp as a 90hp, so this could help I suppose. My spare is in rough shape and it would cost me 70 to get it fixed, if the prop guy would even mess w it. Perhaps a power tech 13.75x13p for Christmas? hmmm. He original motor owner used a 13x21p stainless with some name engraved on the fins, and he was happy w it, but it was no good on the pontoon for obvious reasons. Oh, it was tuned and ported, whatever that means.

I guess if I were to go with a higher motor, a longer set of cables would be necessary. I don't have much $$$ to experiment with. So the prop sounds like the next thing to try since I need a spare anyway. 38% slippage is not good. I wish someone else had tried to raise the motor, just to see if its worth the expense. The folks on youtube seem to feel raising it is a good idea, but who knows what else they did to get their performance closer to maximum. Thanks again for all the help and suggs. C'mon Spring!!!