Top Speed for a new Bentley 18ft w/75 Merc

[beamo]

Hi Guys,

I was wondering if one of you could give me some info on how fast I "should" be going. I just purchased an 18ft Bentley pontoon. I opted for the max allowed 75 hp Mercury 4 stroke. The dealer has been less than helpful since I drove off the lot with the boat. I would like to educate myself a bit for my next chat with them.

The boat will go 20-23 mph at 4500 rpms no matter how many people are on board. I've had 2 -10 adults and thats how fast it goes no matter what. I guess my question is..shouldn;t that engine be revving a bit higher that that? And if so shouldn't the higher revs=more speed? Please forgive the ignorance.

Thanks in advance

-Mike

 

[rickdb1boat]

Re: Top Speed for a new Bentley 18ft w/75 Merc

Mike, are your speeds from the speedometer or a GPS unit? 4500 RPM's is too low. It should hit 5800 RPM's wide open with no problem if lightly loaded. Either you have the wrong pitch prop and/or your Tachometer is reading wrong. I'd have the dealer look at it and posthere what pitch prop you are running. The numbers will be stamped on the end of the prop hub..

 

[5150abf]

Re: Top Speed for a new Bentley 18ft w/75 Merc

More rpm doesn't always mean more speed, it is like down shifting in your car, the engine runs faster but you don't go faster, you will be using a smaller prop with less pitch so the engine will run faster but the boat may not go faster.

But at 4800 you aren't making rated HP so you should have your dealer look at getting it propred right.

 

[beamo]

Re: Top Speed for a new Bentley 18ft w/75 Merc

Thanks for the quick response guys!

The speed I indicated came from my GPS which is pretty close to the speedometer on the boat.

Anyway, I contacted the dealer after posting to get exact details on what he has been trying.

The engine originally came with a 15 prop. This gave me 23-24 mph at 4500 rpm's.
again it made no difference on load. 2 or 10 people the boat went the same speed.

Then he tried an 11 prop. lost about 5mph and engine revved to 6000.

I now have a 13 prop and get around 5000 rpm and 20 mph. I was told by the dealer that the only way to regulate rpm is through the prop. At first this didn't make sense as I thought maybe it was a throttle cable adjustment etc.

I guess it's just hard for me to understand how I could get more rpm's and less speed. I bought the upgraded engine cause I wanted just a bit more speed(around 25 mph) than what I had seen posted and read about. I still think this should be possible. Any thoughts?

I guess the only constant and maybe the added power I get from the 75 say from a 60 hp engine is that load makes no difference. The dealer has been less than enthusiastic about trying to solve my problem but I do have a 1 year warranty and want the boat set up correctly. Thanks so much for any info guys. It is much appreciated. I just don't want to be at the mercy of someone and not know what the heck I'm talking about or what he is talking about for that matter!


Thanks again,

-Mike

 

[5150abf]

Re: Top Speed for a new Bentley 18ft w/75 Merc

Your on your 3 rd prop so I would say the dealer has done his job, that you didn't get the results you wanted doesn't change that, he did all he can do.

You can't prop a boat for speed, only to get the target RPM and what ever speed that is is what you have.

Think about your car, you can go 30 in 3rd at 3000rpm or 25 in 2nd at 4000RPM, it is the same as your boat, the prop is a gear and you want to hit an rpm.

 

[rrhodes]

Re: Top Speed for a new Bentley 18ft w/75 Merc

2" of pitch should have dropped 400 rpm not 1000. I would play with the trim a bit with the 13p prop. I would think you should be hitting around 5500 RPMs and about 22-23mph

 

[MikDee]

Re: Top Speed for a new Bentley 18ft w/75 Merc

That 13" pitch prop sounds about right If the motor is not fully broken in yet, I'd say after at least 2 full tanks of gas, things will change! I don't know about pontoon boat, but most boats benefit by raising the trim angle using power trim at WOT, it gets more of the hull out of the water, Just raise it till your rpm goes up, & your speed picks up, once you hear the prop start to cavitate/ventilate, drop it down a touch, till it grabs,,, That's your best setting! If you haven't tried this, you may gain 500rpm, and some speed.

 

[papasage]

Re: Top Speed for a new Bentley 18ft w/75 Merc

my pontoon has a 3 blade 11 pitch on it . itried a 16 p ss it would onley crowl . if the rpms are at waht the motor should run at wot that is all you are going to git without a bigger motor .pontoons arenot for speed and planning like a bass boat if that is what you wnt you need a deck boat .
my 20 ft. has a 40 hp and its top sped is 16 mph . you wan`t git that much more with a bigger motor .
just prop till you git max sujested rpm and let it go .

 

[branman1971]

Re: Top Speed for a new Bentley 18ft w/75 Merc

I have a 20ft pontoon boat with a 70hp 2stroke. With a 13.75x15 prop i only got 15mph with 4200rpm WOT. I went to a 13.75x13 prop and I now get 5000rpm at 18mph WOT. I know I should maybe drop down in pitch again, but this is just reference for you Beamo.

 

[MikDee]

Re: Top Speed for a new Bentley 18ft w/75 Merc

A Hydrofoil, or a Doel fin may help you get more boat out of the water, and on plane quicker for tubing.

 

[Silvertip]

Re: Top Speed for a new Bentley 18ft w/75 Merc

Your numbers with the 13P are not very far from correct. You have a pontoon, not a planing hull and this boat will not be a speed demon. Yamaha test of a G3 pontoon showed 25 - 26 MPH with their 75 HP four stroke, a 13P prop and 2.31:1 gear set which compares very closely to your Merc. The speed difference is because the G3 pontoons are shaped differently and are therefore faster. On a Weeres pontoon, that same engine showed a tick over 21 MPH.

 

[rrhodes]

Re: Top Speed for a new Bentley 18ft w/75 Merc

I disagree that you need a deck boat if you are after speed. What I will say is that in order to get speed out of a toon you need to buy a performance toon and a lot of HP. A 26' JC Tritoon 225hp is designed to run 45mph+ but they will cost $40k+ Manitou, Bennington and even Bentley all sell performance 200HP+ toons that run 50 - 60+

These boats take a lot of energy to move and with anything speed = $$$

 

[beamo]

Re: Top Speed for a new Bentley 18ft w/75 Merc

Many thanks for all the help!

So it looks as if i'm in the ballpark of my expectations. But it still seems I'm not getting the rpm's I should be. I knew I wasn't going to have a fast boat and wasn;t expecting speeds in excess of 30 mph.

Any Ideas on how I might be able to get that last 500-800 rpms out of the motor. Or should I settle for 5000 rpm wot?

I would guess the extra rpm the engine should be turning would give me the 22-24 mph I was told to expect from the dealer. I know he has changed the prop twice on me but for over a month he told me it needed to break in. I had put about 6 tanks of gas in it by then and 20 plus hours of use and still was reluctant to change the prop. It was only after being a pain in the butt that he sent a kid to swap out props. This has been since I picked the boat up in May and he is just now trying to fix it. I don't think it my responsibility to go to a website and tell him that it should be running at 5-6k. He told me 4500 rpms were fine and I guess hoped I'd go away. So though he in fact swapped out the prop twice thats all he has done in 4 months and then only swapped out the prop and left it to me to see if it was ok. Not a good idea as i didn't know the differnece between prop pitches and diameters etc. All I knew was that the salesman told me to get the 75 hp and I would get about 25 mph. And he has been damn near invisible to me since I signed the papers.

Anyway thanks for setting me straight about the how rpm's and speed relate as far as props go on an engine etc. At least now I understand the forces at work. Very much obliged to all who threw in their 2 cents.

-Mike

 

[rrhodes]

Re: Top Speed for a new Bentley 18ft w/75 Merc

Next time out bring her up to WOT. Trim the motor up until you hear the prop ventilate then drop it just enough to get the ventilation to stop. That should be your optimal trim. That should also bring your RPM's up a bit.

 

[branman1971]

Re: Top Speed for a new Bentley 18ft w/75 Merc

Mike,

I am no pontoon boat GURU, and I learn something new every time I go out, but I must tell you that I was turning 4200-4500 rpm WOT, and I had 2 local boat shops in my area, as well as 2 good friends of mine, and also lots of folks on this forum tell me that I would considerably limit the life of my motor running those rpm's at WOT. And you know, i'm starting to believe em...

Hard to think that your boat salesman would advise just to run it like it is. Sounds like he needs a different career field and I would talk to the owner about your whole boat-buying experience. As for the motor being ok with those low rpms at WOT, you tell 'em I have the phone numbers of 2 reputable boat shops in my area that will beg-to-differ.

 

[MikDee]

Re: Top Speed for a new Bentley 18ft w/75 Merc

I would say that 13" pitch prop is about as close as you're gonna get, but as I mentioned most boats benefit from a Doel Fin hydrofoil, and also from powering up the trim to just before cavitation begins (you can usually hear, & feel this) when at Wide Open Throttle (WOT), each of these usually gets more boat out of the water, less hull drag, while gaining up to 500rpm, But I don't know how well this apply's to pontoons? Good Luck, Mike

 

[DMAN1968]

Re: Top Speed for a new Bentley 18ft w/75 Merc

If you are having no prop ventilation issues at all then you might would see a performance/rpm increase by raising the level of the motor on the transom. Not just using the trim, but by physically raising the motor up a hole or two on the transom mount. This usually gains some rpms and speed due to having less of the motor in the water and thus less drag.

This really is only an option if you are not having any ventilation issues as it woould probably make those worse. I agree with the doelfin possibility to. Would probably make a significant difference on that 18 footer.

 

[MikDee]

Re: Top Speed for a new Bentley 18ft w/75 Merc

What DMAN said is also true, on your average boat, with the cavitation plate paralell to the bottom, you raise the motor till the cavitation plate is even, or just above the bottom of the hull for best performance, that is kind of hard to figure on a pontoon, but yes, if you raise it high enough so that it's almost at the waters surface when on plane, and doesn't ventilate, or cavitate, that's your optimum setting.

 

[quantico]

Re: Top Speed for a new Bentley 18ft w/75 Merc

I put a 40 hp evinrude on my older rivera cruiser 21 foot pontoon boat. I also added a cmc power tilt as I live on a shallow river and want to keep the lower unit dry and clean when not being used. I built a new transom and support system and welded it up with half inch aluminum as the back plate that all mounts to. I can tell you a couple things about ventilation and getting the prop to hook up because it took a lot of effort to get that done. I also just went thru the same issues getting my neighbors 50 hp mercury hooked up on his 24 foot pontoon.

Based on my experiences there are three critical points in getting a pontoon boat setup correctly.

1. the engine needs to be mounted in a way that can get the engine near the lower unit a few degrees forward of perfectly vertical. That is when the engines lowest part ( lower unit and prop ) is tilted forward slightly. I had first mounted my engine so that on dry land sitting perfectly level the engine was exactly vertical when fully down. Of course with passenger loading and the heavy engine and heavy power tilt the back end of the pontoon boat was starting several inches lower than the bow. What I found was that the prop liked cleaner water not disturbed by the transom bottom / which has a splash bracket mounted at the bottom. Moving the engine by shimming the engine mount with about 3/4 of an inch of washers on the top mounting holes got the prop into clean water. Most transoms for pontoon boats are at a pretty good angle to get the engine mounted prop quite forward, this is needed.

2. I lowered the engine a couple inches so that the anti ventilation plate is quite a bit under water with the boat stopped. This helped keep the prop hooked up on turns so that the engine does not over-rev and lose contact with the water. Since I live on a shallow river I decided to raise the engine up one hole this spring to try to get an inch of clearance back for the skeg and prop. I introduced a small amount of loosing the prop hookup when on sharp turns. I will lower the engine back to the position that worked well last year. ( this was a pretty low level for the engine ) . I do get lots of splash and water thrashing near the engine, but I am getting that at 18 mph now with an engine that is not ventilating. Moral of the story is that the engine comes up quite a bit / sort of up on plane with the rest of the boat at speed. You need the engine mounted low enough that it is at the right height when the boat is lifted up on plane.

3. balance is critical on a pontoon boat. I went from a 25 hp engine to a 40 , and added lots of heavy metal in the transom along with a cmc power tilt unit. This was getting the boat way to back heavy to handle well and turn well. I moved the batteries to the front and wired them with 2 gauge battery cables. I also moved the fuel tank to the front of the boat. I then moved the steering console forward another foot - 18 inches to move the driver and passenger weight forward a bit. This was countered a bit adding GPS and depth gauge and a stereo / amp / speakers. All in all moving as much weight forward as I could helped a lot. You sure don't want a nose heavy pontoon boat, but in my case I did not want a butt heavy boat either. I installed a level bubble on top of my steering console so I can see slight changes in port to starboard weight shift and can see the bow to stern loading with the boat at rest and up at speed. That level bubble is a very important safety item and I highly suggest getting one if you don't have one. Frankly I have never seen another one, so my guess is that you could buy one and learn a lot about how to balance weight on your boat. Having the boat balanced well makes it safer and hook up better to the water with the prop.

Hopefully this will help you in getting your pontoon boat working better for you.

 

[beamo]

Re: Top Speed for a new Bentley 18ft w/75 Merc

Hi Guys,

Thanks again to all who posted. Well I was able to get out this week and I just can't seem to get the rpm's above 4900 - 5000. I tried playing with the trim and didn't get any appreciable results before cavitation. I'm still running about 20-21 mph with 9 people on board. My concern now is if I am hurting the engine by only getting about 5000 rpms wot?

Much obliged again!

-Mike