My dad just got an 18' pontoon boat with a 70hp Yamaha 4 stroke. Per Yamaha, the specs are 70hp@5800 rpm with an engine operating range of 5300-6300rpm. Currently, it has a 14-11 prop and has a top speed of 21-22mph. To me, that is a hole shot prop (akin to 4.11 gears in a car where you increase acceleration by sacrificing top speed)it easily and freely revs to the 6300rpm max engine speed with the throttle lever only about 3/4th of the way forward.

My dad wants to increase top speed even If that slows down the hole shot. By increasing the prop pitch, how much could we realistically increase the top speed? I'm sure it's a bell curve. Increasing the pitch will eventually bog the engine down. I've been on boats that are the opposite where WOT is only 4000rpm, well below the engine's max rpm, as the pitch is too steep and the boat is slow accelerating and has a slow top speed. My dad was thinking of going to a 13 pitch. Just want to get an idea of the maximum mph we could hope to increase. Would a different prop get him to 25mph?

I really have no experience with pontoon boats but I can foresee where a pontoon boat would be harder to plane (does a pontoon boat actually plane?) than the same size mono or cat hull and so the smaller pitch may be required. One option is to go with the larger pitch and then have a prop guy put more "cup" in it so that you get more bite when taking off.

Roughly a 13 would get you to 25...a 15 to 27mph.

call a prop manufacture, they will have exactly what your looking for, when I got a new prop for an alumacraft shallow water boat I wanted a hole shot prop, I called the prop place and they gave me a list of options that would work for my engine, purchased the prop and they were 100% correct, that boat would pop up in no time.

so I basically switched from a 3 blade "speed prop" to a 4 blade "hole shot" prop with the correct pitch for my boat and engine..

the prop guys know what you need

Roys Machine and Fabrication Inc. | Facebook i think I called this guy in Aransas Pass

I spent months looking for the proper prop for my pontoon house boat.
The solution was a four blade spitfire from Mercury.
The blades were so big they looked like elephant ears.
It took several tries to get the right pitch. My dealer let me take several to find the proper one.

Don't think you're gonna get much else out of a 70 horse motor, but not to say you can't optimize it a tad.

It's going to depend entirely on if the pontoon is planing versus just reaching maximum displacement speed. With a 70 it would be hard to tell. That would plane some lightly loaded pontoons but wouldn't some others given weight. Then you have to consider weight is highly variable.

The pontoon hulls will have a maximum design displacement speed and driving it faster than this will take a lot more power unless you can make the transition to planing. It is getting through they transition without uselessly churning up water and wasting fuel that is the issue, and with a 70 on an 19 foot pontoon, depending on load, that's going to require some fine tuning and possibly different props for different passenger loads.

Cstew.....In my daily work I probably spend a portion of everyday having a end user enquire about his prop and performance on his boat. As a manufacturer we spend tons of effort on testing and recommendations of the proper prop for all of our products with a given outboard.
Props are very complicated and prop designs do different things on different hulls and planning surfaces.
The manufacturer of the pontoon boat should have a prop and engine set up data chart for every product they build. Dealers can be very lax at making sure that the correct prop is on the products they deliver. We deliver all our products standard with the correct prop for each set up.

With that being said you want to turn your Yamaha around that 5800-6000 max RPM range. That would be at WOT (wide open throttle), best trim angle, and the normal load you are going to use the boat with fuel and equipment. Correct engine height is critical in prop selection. The pontoon manufacturer knows what hole the engine should be mounted on. Before you go any further on looking for a new prop make sure that the engine is mounted at the correct height or you will be wasting money and time.
If the engine is too low it will cause it to have too much drag and will bog the engine. If it is too high it will cause prop slippage and cavitation and allow the engine to turn too much RPM.

Once you have determined and made sure that the engine is indeed mounted at the right height you can start your evaluation of propellers.

I would caution you about using a "prop shop" to tell you what you need. After 37 years in the high performance marine business I have only found one prop shop in Texas that truly knows what they are doing.

Putting "cup" in a prop is common term people will use but you can add extra cup to a prop on the leading edge and it will help in making a faster "hoe shot" but putting cup on the trailing edge increases bow lift and cuts down WOT RPM. "Cupping" a prop on the trailing edge will help raise the bow of a performance boat, freeing up the running surface and giving if more bow lift...thus increasing speed.

A pontoon boat does not need bow lift....

Also diameter of the prop is very important in prop selection. Going to smaller diameter prop and higher pitch will oftentimes accomplish nothing in performance gains.

From what you are saying it does appear initially that you could go up two inches or so in pitch. Rule of thumb is that one inch pitch will equal 1-2 MPH gains IF you can still turn it at 5800-6000 RPM.

Yamaha has "Performance Bulletins" on their web site for lots of boat motor combinations. Yamaha owns their own prop company and they recently bought Turbo props and they spend tons of money doing research and testing to offer and recommend the proper prop for the boat builders they do business with.

I highly suggest you contact the pontoon company first and ask them what they recommend for your set up. Any company worth their salt should be able to exactly give you correct engine height data and the suggested prop/props you should use.

That will be the simplest solution for you. You can get as many opinions on props and what to use as asking what is the best pistol to own........... and waste lots of time and money chasing that rabbit.

Call Nettles Props in Austin. Great staff, very knowledgeable, great selection and price.

Typically going down in pitch number will increase holeshot but decrease top end. So going from a 14 to a 13 would decrease top end.

Here is a prop speed formula we use as a guide at work. It is usually fairly close on a high performance hull with a prop being very efficient with little slip:

RPM x PITCH x Gear Ratio of lower unit. ( Must be a decimal figure...IE Yamaha 70 is 2:33 to 1 = .43 ) x .0009461 (Constant) x Prop efficiency (Typically 80-95%) = speed

So 6000RPM x 11" pitch x .43 x .0009461 x.85 = 22.8 MPH ---what OP says he is getting now

6000RPM x 13"pitch x .43 x .0009461 x .85 = 26 MPH. ----- what he might get with a 13" pitch

On a pontoon boat the slippage may be closer to 80% than 85% bringing those actual speeds down somewhat.
Performance hulls can achieve 90 - 97% efficiency on a clean hull.

We have a current boat that has a Yamaha ShO 250....gear ratio is .57.
Turning a 28" pitch 3 blade 6000 RPM.
28 x 6000 x .57 x .0009461 x .90 = 81.5 MPH GPS shows 80.5 so pretty close to formula.

Think as listed he has a14" diameter by 11" pitch.

I Second Propellers by Nettle in Austin. Ask to speak to JT Nettle. Man is so knowledgeable that the prop manufacturers use him as a go to as well.

?? Prop manufacturers?

Merc, Power Tech, Acme, Johnson Propellers

These I know if, but not limited to

Just wondering. I worked for Mercury and Quicksilver division... also worked close with Turbo, Protestor, Yamaha and Second Effort of old OMC Raker prop days but did no business with Nettle. Never heard of Acme or Johnson in my world.
Have known Nettle for many years. A good prop shop.

pablof said:

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I Second Propellers by Nettle in Austin. Ask to speak to JT Nettle. Man is so knowledgeable that the prop manufacturers use him as a go to as well.

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Nettle got a van that drives all over picking up and delivering props.

So does Baumann Propellers in Houston.
My dealers use him a lot on high performance props and he is good as well as far as prop shops go.

Yeah those guys that do that are master craftsmen.

Yep. There are a few good ones but also a lot of prop shops that don't do good work and lead folks astray and cost dealers and manufacturers a lot of time and money by telling folks bad information and selling them wrong propellers.
In 37 years I have dealt with only a couple in my career that really know what they are doing.... imho.
I am very careful of who I recommend because of those past experiences.

I have also had many guys tell me of Baumann doing great work and knowing their stuff. We have personally used Nettles in the past and were very pleased.

Now I just need to find a 5 or 6 year old Shallow Sport 21 Sport or Majek Illusion with a Yamaha 250 SHO and I will be ready to pull the trigger.

Check out this innovative product specially designed to improve pontoon boat performance.

Hydrofin is essentially a hydrofoil system mounted to the toons under the boat. Significantly improves top speed, fuel efficiency and more importantly ride and handling. Basically gets the boat out of the water. I highly recommend.

https://shophydrofin.com/