Eight Questions About Propellers for a 170 MONTAUK

Optimizing the performance of Boston Whaler boats
bikerGus
Posts: 14
Joined: Mon Aug 14, 2017 9:28 pm

Eight Questions About Propellers for a 170 MONTAUK

Postby bikerGus » Thu Oct 27, 2022 12:54 pm

My boat is 2006 Montauk 170 with a Tohatsu 90-HP. I’ve been experimenting with various four-bladed propellers from 13 x 19 to 13 x 15. Ultimately I chose a 13.25 x 13. My assumptions are lower pitch:
  • reduces top speed
  • improves [acceleration from a standing start at full-throttle]
  • improves trolling speed
  • improves docking maneuverability, and especially with a heavy load of people and fuel aboard.

Q1: are my assumptions valid?

Q2: are changes [in boat performance caused by changes in propeller pitch and diameter] too subtle that the effect is probably not noticeable?

Of course the 7-MPH lower boat speed at full-throttle is noticeable, but a top boat speed of 35-MPH top speed is okay for me.

Q3: can a stainless steel propeller be taken to a prop shop and re-pitched?

Q4: how is re-pitching of a propeller done?

Q5: is a propeller re-pitched at a propeller shop by putting the propeller into a propeller re-pitch machine?

Q6: are the propeller blades bent during re-pitching

Q7: do you recommend having a propeller re-pitched?

Q8: do you recommend against propeller re-pitching and recommend buying a new unmodified propeller?

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Phil T
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Joined: Thu Oct 22, 2015 6:08 pm
Location: Was Maine. Temporarily Kentucky

Re: Finding the optimum prop pitch

Postby Phil T » Thu Oct 27, 2022 2:03 pm

B -

Prop selection is alot of math and a dash of art. It is far from straight forward or simple.

Before i address your specific questions, let me state a few important facts.

- Size is not universal. Stating a size without a make/model is worthless as all models of prop have a unique blade design and perform differently that others in the same size from another model.

As for your questions -

Q: ...lower pitch reduces top speed but improves hole shot performance and trolling speed/docking maneuverability, especially with a heavy load of people and gas.

A: Basically correct.

Q: ...can a ss prop be taken to a prop shop and “re-pitched”? If so, how is this done, stick it in a re-pitch machine and bend the prop blades? Is this advisable, or better to just buy a new prop?


A: Yes, a prop is typically repitched by up to 1". This should be done only when there isn't a model in the desired pitch. In my experience 95% of of repitch instances are not the appropriate course of action.

Over the last 20 years Boston Whaler owners have been repowering their boats and installing new propellers. With many owners testing several props before finding the appropriate one. Much of this testing information has been captured.

For a Montauk 170 and a Tohatsu 90 hp (gear ratio of 2.08 and a WOT rpm of 6,000) there are two recommended models of props. They are both 3 bladed.

What is the intended use and typical load (be specific) in the boat that requires a 4 bladed prop? As you mentioned, there will be a loss of WOT speed with a 4 blade versus a 3 blade.

I would suggest you have the wrong prop model and size as 13" pitch is too low.
1992 Outrage 17
2019 E-TEC 90
2018 LoadRite 18280096VT
Member since 2003

jimh
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Location: Michigan, Lower Peninsula
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Re: Eight Questions About Propellers for a 170 MONTAUK

Postby jimh » Fri Oct 28, 2022 5:05 pm

bikerGus wrote: My assumptions are lower pitch:
  • reduces top speed
  • improves [acceleration from a standing start at full-throttle]
  • improves trolling speed
  • improves docking maneuverability, and especially with a heavy load of people and fuel aboard.
Q1: are my assumptions valid?
Yes.

bikerGus wrote: Q2: are changes [in boat performance caused by changes in propeller pitch and diameter] too subtle that the effect is probably not noticeable?
No.

bikerGus wrote: Q3: can a stainless steel propeller be taken to a prop shop and re-pitched?
Yes

bikerGus wrote: Q4: how is re-pitching of a propeller done?
By a skilled artisan by hand.

bikerGus wrote: Q5: is a propeller re-pitched at a propeller shop by putting the propeller into a propeller re-pitch machine?
No

bikerGus wrote: Q6: are the propeller blades bent during re-pitching?
Yes

bikerGus wrote: Q7: do you recommend having a propeller re-pitched?
Only if the pitch change is one inch or less.

bikerGus wrote: Q8: do you recommend against propeller re-pitching and recommend buying a new unmodified propeller?
I usually buy used propellers that are in excellent original condition. Reworked propellers are likely to be a bit suspect, depending on the reputation of the shop employed to do the work.

jimh
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Location: Michigan, Lower Peninsula
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Re: Eight Questions About Propellers for a 170 MONTAUK

Postby jimh » Fri Oct 28, 2022 6:55 pm

bikerGus wrote: I’ve been experimenting with various four-bladed propellers…
You have been sucked into the four-bladed-propeller vortex of craziness.

What made you think a 90-HP engine on a small boat like a 170 MONTAUK needs a four-bladed propeller?