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[color=#000066]Hallo everybody from Italia. I'm going to power a 1990 Montauk with a Honda 90 Hp. I have carefully read the performance reported in the BW site on the Montauk 170. They are with a Mercury 90 Hp 4 stroke.
Speed 42,8 Mph, Prop Laser: II 3 x 13 1/4 x 20; RPM: 5.900 (power range of the Mercury engine being 5000 - 6000); Transm. Ratio: 2.33:1; Boat Weight: 2291 Lbs.
I used the above data in the speed formula and I have found out that the regression is more then 10%.
I think this is a very high value, at least for performance professionally measured by a manufacturer. Am I wrong ?
For this reason I'm in doubt on what prop use with my Honda.
The Transm. ratio is the same (2.33:1), the max power range is 5300 - 6300 RPM and ... the power of 90 Hp is the same and I was indeed considering to choose a 20".
Among Honda Solas props I was considering a 4 blades in steel, HR TITAN, either 13 x 19 or 13 x 21 (no 20 available in the range).
In the HR TITAN 3 blades, the available sizes are: 13 7/8 x 19 or
13 7/8 x 21.
I would prefer the 4 blades solution that - in my experience/opinion - is offering a better time to plane and a smoother run.
But looking at the result found in BW web what do you suggest ?

From one side, the '19 would match with the higher RPMs which Honda has, from the other side if the regression is in the range of 10% I'm going to loose some speed.

From the other side the '21 would allow anyway some more speed (even in the case of 10% regeression), considering that the Montauk tested was loaded with 500 Lbs., and that the Honda 90 Hp engine is known to give some few more hp against the mercury 90 Hp.

Because I'm afraid that the best pitch is 20 ... I ask you if the Mercury Props are perfectly suitable for my Honda (fit without accessory) and - in such case - which would be the best steel 4 blades prop in the Mercury very wide range.

I'm very sorry for my many questions. I promise to post the results I will reach with your help.

MANY MANY THANKS TO ALL OF YOU WHICH WOULD GIVE ME DIRECTIONS BASED UPON EXPERIENCE.

Ciao from Italia [/color]

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Hello Punta
From what I have read 10% slip (regression) is within the range of what is considered a good value. My understanding is that a prop has to balance slip and friction. Too small prop surface and the slip is too high, too much prop surface and the friction is too high. Either one is inefficient. My description is very simple and does not account for prop design. I'll let the experts discuss that. I'm sure they will be along soon.

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Punta,

The older style Montauk (and most 17's of that era) are lighter and a tad smaller than the new 170. Maybe as much as 500 LBS. another factor to consider.

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Thank you both for your kind answer.
Mi first name is Vincenzo (family name Fratello).

Punta Licosa is the place where I go fishing since I was a kid.
If I understand how to do it, I will post a pic of the Island.

I'm not an expert on props maths even if I read some where then 5% slip vs. the theorical speed is achievable in "best conditions".

The 500 Lbs. was not the delta weight between the 17 and 170, it was the load of the boat under testing, on top to boat and motor weights.

I look forward to hear from the experts, as you said !

Thanks again and ciao from Italia

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We put a Honda Titan Solas 13 1/4 15R Stainless Steel Prop on our new Honda 90 at the recommendation of the Regional Honda Rep as we talked about it during installation in the shop. No problems or complaints...runs fantastic. Good Luck

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Thank you very much for your input.
I presume you go boating with heavy load, don't you ?
As matter of fact, the max speed formula - for a so short pitch (15) is giving 33,376 knots (or 38,42 miles per hour), @ 6.300 RPMs.
If we consider a minimum slip of 5%, then your max speed would be 36,5 miles per hour, assuming your motor is running at 6.300 RPMs.

Is this kind of performance you are indeed getting with your boat ?

Last not least, is your prop 3 or 4 blades ?

Thanks, all the best,

Vincenzo