CONTINUOUSWAVE

[If you visit the photograph in the URL given below you can see an engine mounted one-hole up which the author says is] how the dealer mounted the SUZUKI 70-HP outboard engine on a 1992 Boston Whaler MONTAUK 17. [The engine mounting height] seems to need to be [raised] one additional hole up. The engine speed was 5,500 RPM when the boat speed was 32-MPH.

Should [the aluminum propeller be changed to a propeller made of] stainless steel?

http://i240.photobucket.com/albums/ff24 ... sjgfcd.jpg

[This article has been moved to PERFORMANCE.--jimh]

To determine if raising the mounting height of the Suzuki 70-HP engine on your 1992 Boston Whaler MONTAUK to two-holes-up from its present position at one-hole-up will improve the performance, raise the engine to the higher mounting position and test the boat. No one can really predict how well you and the boat will like either mounting position. I think often some rationalization is used to stick with a mounting position, either a higher one or a lower one, because moving the engine up and down is a bit of work.

In general, one can say that raising an engine mounting height must be done with care and an eye on the cooling water pressure. If there is any sign in a reduction of cooling water pressure as engine height is raised, there is risk of insufficient cooling for the engine. The risk of insufficient cooling for the engine is of such concern that it overrides any possible benefit--if indeed there were some significant benefit--to be gained by using very high engine mounting height in a small center console boat whose top speed is less than 40-MPH.

Very high engine mounting height is usually only seen in boats that operate above 60-MPH, and those boats often employ specialized water pick-up devices to improve engine cooling.

As for changing to a propeller made of stainless steel from one made of aluminum, the benefits and drawbacks are well known. See Part-2 of my article on propellers which discusses propeller materials.

As for evaluating your performance data, without knowing the gear ratio of your engine or the rated pitch of the propeller an informed opinion will be hard to find, but at first glance I'd would have thought a MONTAUK with 70-HP would be faster, closer to 40-MPH.

Data on performance of a MONTAUK with various horsepower is presented in an article in the REFERENCE section in the series of articles about the 17-foot hull. See

http://continuouswave.com/whaler/refere ... power.html

There are also dozens and dozens of comprehensive reports of MONTAUK performance in the archives of the old forum. You can find them with a search by starting at

http://continuouswave.com/whaler/oldForum/

More than likely you'll need to raise motor by one hole. That may increase rpm when engine is trimmed up to maximized speed.
If you then are over-revving (whatever the maximum rpm of your engine)' the you should be able to move to a higher pitch prop.(probably by 1-2"). Stainless steel props are generally preferred, due to it's strength, durability and thinner, more efficient blades.
Your goal is to have the anti ventilation plate(just above and behind the prop) to be ABOVE the bottom (keel) of the hull, and your rpm close to your maximum allowable rpm, and the boat loaded as you normally wood, engine fully trimmed out, with hopefully a stainless steel prop.

I would expect your top speed to be in a range of 35-39 mph.

I agree with macfam.

Up one at least, if not 2. Note you will gain 150 rpm's at WOT for each hole you go up.

Yes, stainless gives better performance.

Warning, prop size is not universal across manufacturers and or models so don't run out and get the same size you have now.

What is the make, model and size prop you have now?

The engine is already mounted one-hole up. Look at the image of the engine mount in the first article posted.

Jim. Up one more?

No brand name on the prop but it is alum and stamped 13 1/2 x 15

I would like a stainless. Which one?

Suzuki WOT is about 5800 rpm

Orchemo wrote:Jim. Up one more?

See my earlier reply.

You are now "1 hole up"

I say go into the "2 holes up" position if not higher.

Thanks Phil. Will move it up this weekend and give it another run.

You did not supply information about the gear ratio of your outboard engine. The Suzuki website shows their current model F70 has a gear ratio of 2.59:1. Please comment on the gear ratio of your engine.

Moving ahead on the assumption that your Suzuki DF70 engine has a gear ratio of 2:59:1, we can now review the performance data you have supplied. You have given the following information:

RPM = 5500
RATIO = 2.59 (presumed correct from the Suzuki specifications)
PITCH = 15
SLIP = (to be calculated)
MPH = 32

Using a propeller calculator, we find the SLIP value for this data to calculate to

SLIP = -6

When a SLIP calculates to a negative number the general assumption is the propeller pitch was understated. We adjust the pitch upward by 1-inch, and recalculate.

RPM = 5500
RATIO = 2.59
PITCH = [16] (adjusted)
SLIP = 1
MPH = 32

This calculation shows that your propeller is really working quite well. Generally one wants to see the SLIP value below 10 when the boat is running on plane at full-throttle.

The gear ratio is quite a high reduction, somewhat typical for four-stroke-cycle smaller engines. Let's plug in some other values for PITCH, let RPM remain at 5,500, use a SLIP value of 8, and use a target boat speed of 37-MPH. A target speed of 37-MPH seems quite reasonable for a MONTAUK with 70-HP. Now we let the calculator give us a PITCH value for the propeller.

RPM = 5500
RATIO = 2.59
PITCH = (to be calculated)
SLIP = 8
MPH = 37 (a target speed)

PITCH calculates 19.8-inch. This will give some guidance about what value of propeller pitch to try next.