average mph increase per pitch?

[texasfishon]

need advice, i want to buy a new blue wave 180 v-bay. it weighs 975 empty dealer says put a 90hp yamaha on it but the boat is rated for 115hp the difference in cost is $1000, the only info i can find compareing these two motors on that boat shows the 115 with a 22 pitch prop = 51mph @ 5300rpm and the 90 with a 17 pitch = 41mph @ 5600rpm recomended rpm is 4000-5500. question is how much more mph will a 19 or 21 pitch prop give the 90 and/or is the 115 worth the extra $1000?<br />thanks in advance for any wisdom.

 

[JB]

Re: average mph increase per pitch?

There are too many variables involved to give you a good answer, TFO.<br /><br />The correct prop for any package is the one that will allow the engine to reach, but not exceed, the top rpm in the operating range at WOT (wide open throttle). Speed is incidental to prop selection, rpm is the deciding factor.<br /><br />To go faster than that, you would need more power and a different prop. That is why the 115 gets a 22" and the 90 gets a 17", on the same boat.<br /><br />Hope that helps. Welcome to iboats.

 

[LubeDude]

Re: average mph increase per pitch?

I would go for the 115 if it were me and I could swing it, I have never seen anyone wish they had "less" HP when it came to a boat. You will be glad you have it if you were to get stuck with a storm coming in and you need to head for home. Also, I believe you will get actually better economy do to being able to power down slightly at a cruising speed thus saving fuel. (Important with fuel prices as they are).<br /><br />As to your original question about how much speed per pich, Thats a tough one. I do not think that you can accuratly give an answer to that question because sometimes you do not get a gain with a pich change and sometimes you do.

 

[cobra 3.0]

Re: average mph increase per pitch?

10 mph is quite a difference. I would go with 115.

 

[DJ]

Re: average mph increase per pitch?

There is no comparison between the 115 and the 90 Yamaha. The 90 is a three cylinder and is at the top of its capability putting out 90 hp.<br /><br />The 115 is a much larger engine and is understressed. Because of the larger displacement, it has more usable torque and can swing a larger prop.<br /><br />For $1G, get the 115.

 

[Dhadley]

Re: average mph increase per pitch?

An underpowered boat can be as dangerous as an overpowered one.

 

[teamstromer]

Re: average mph increase per pitch?

texasfishon,<br />i recently purchased an 18ft aluminum boat weighing it at 1275 lbs, it has a merc 90hp 2stroke. i was going to advance to the 115, dealer said all i would gain is 5mph more and he would charge me $1500 for upgrade. he told me how merc engines provide torque to the prop no the shaft and the 90 would be enough. i researched this and dealer was correct. i am pleased with the 90.<br />good luck<br />teamstromer

 

[teamstromer]

Re: average mph increase per pitch?

PS from teamstromer,<br />my boat maxed out at 45mph with 3 people on board. 45 is pretty fast on the water. the rpm's are the important factor.<br />teamstromerxx

 

[quantumleap]

Re: average mph increase per pitch?

I think the money difference will pay for itself if you ever resell the boat. I agree with LubeDude. I don't think I've seen many posts from people contemplating downsizing motors because they have too much power!

 

[KCLOST]

Re: average mph increase per pitch?

My experience, shows on average a 5mph increase in speed with every 1" of pitch increase. THIS IS UP TO A POINT... After that the WOT rpms drop to the extent that speed is comprimised dramatically.<br /><br />My boat will run 55mph with a 23p and 60-61 with a 24p... rpm's drop about 200 though.<br />I'm still doing some testing, getting ready to try the 24p after an engine lift, and also a 25p...I'll let you know what I find just for reference....

 

[youngboater]

Re: average mph increase per pitch?

I would go for the 115. I have a 16' w/75 Mercury and the max HP is 90. At times I wish I had that 90. It does not hurt to have extra HP. As long as you are not exceeding the MAX recommend HP by the U.S. Coast Guard.

 

[Jdeagro]

Re: average mph increase per pitch?

I will not get into the debate over more or less HP, but HP is not the only thing that determines top performance. It seems that the boat with the 90 was running a bit above the recommended RPM range and could use a bit more pitch. However the performance of the boat should be addressed by a combination of boat hull efficinecy, and prop, and HP. Obviously the payload, and expected use is a personal factor which should also be a consideration.

 

[Chris1956]

Re: average mph increase per pitch?

Texas, I doubt that the difference in the power of those motors will give you a 10 MPH increase. I think it will be less (~5MPH). However, there is value in having the extra horspower (provided you do not use it all the time) since the bigger motor will not have to work as hard as the smaller one. You might even save a little gas (for equal speed) if you run the 115 HP motor at 2/3 throttle, versus the 90HP motor at 7/8+ throttle.

 

[Hooty]

Re: average mph increase per pitch?

Using a gear ratio of 1.87:1, the 115hp engine turning a 22" pitch prop 5300rpm and moving 51mph, has a prop slip of 13.6%. Using the same figures except a 23" pitch prop, the boat will be moving 53.3mph.<br />1" pitch = 2.3mph (mas y menos)<br /><br />c/6<br />Hooty