MacGregorSailors.com

just a quick question for you motor heads. We took delivery of our 05m in Seattle, played in the puget sound for awhil and then we brought it home. Have sailed the boat and love it overnighted on the boat and loved it motor the boat and also loved that. ok so heres the question at sea level the boat with the 70 suzy would motor at just over seventeen knots at wot here at 5000 ft the best i can do is just under 12 is this a result of altitude or is there something else I should be looking at

I'm guessing it's an altitude effect. I've been messing with different props on mine (Tohatsu 40 HP carburated) to get more speed at 8000 feet. I went to a different prop that got me to over 5000 rpm at WOT but the boat speed actually went down.

Moe might have something to say on this.

If the Suzuki DF70 EFI is anything like the DF50, it takes a reading off the Manifold Absolute Pressure sensor as soon as you turn the ignition to ON, but before you get to the START position. That way it reads the atmospheric barometric pressure, and can compensate for the lesser oxygen content at higher altitude. It does this by metering less fuel to the engine than it would at sea level. Less oxygen + less fuel = less power. You're probably losing at least 15% or 10HP.

That doesn't totally explain that much drop in speed. The difference in the bouyancy between salt and fresh water explains some, but what we need to know is what the prop diameter and pitch are, and what the engine speed in rpm was/is at the two boat speeds you mention. We also need to know whether one was ballasted and the other unballasted, and how much "stuff" you've taken aboard since you got the boat home.

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Moe

I purchased my 97' X near Baltimore on the Chesapeake. It's a Nissan 50 PLUS. I had the motor checked out and tuned. Here at about 3500 feet I can only get 4000 RPM's at wot. The prop is the stock unit that came with the motor. Shouldn't this motor wind up to 5000 rpm at wot?
Moe, what am I missing? This motor runs like a champ and the shop that did the tune said it was in perfect shape...
Dik...

Dik, if they come with a prop from the manufacturer, 40-60HP motors usually come with the right prop to push a 1,000-1,500 lb fully loaded 14-16' fishing skiff. It doesn't give the motor enough "leverage" to push a 3,000-3,600 lb MacGregor (much less one at or near 5,000 lbs with water ballast). As a result, the engine, even if it's tuned and running perfectly, can't accelerate much past its torque peak. And at 3,500' ASL, it's producing less than the horsepower it's rated to produce at sea level, even if it is well tuned and in perfect shape. So you need an even lower pitched prop than you would to achieve rated rpm with a MacGregor at sea level.

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Moe

Thanks Moe,
I'll check that out this weekend... I think the pitch and diameter are stamped on the prop...
Dik...

Moe,
If Dessert Wind bought the boat from BWY, which is likely since they are the only dealer in seattle the prop is a 13 3/4 X 13.

Thanks, Tom. Now I remember yours and Frank C's comments, and our PMs, about whether the DF70 should have an 11" or 12" pitch prop instead of the supplied 13" pitch. I looked back and noted you're only getting 5,150 rpm WOT with the 13" and that's at sea level in saltwater. If you're overpropped under those conditions, desert wind most certainly is at 5,000' and freshwater with a 13".

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Moe

Moe, Tom is correct I did pick the boat up at bwy in seattle I will check this weekend on wot rpms I dont think I am turning 5000 at the lake.

moe just got back from the lake and lo and behold the rpms on wot is just barley over 4000. and the best speeds are 9 knots whats next is a trip to san juans on the 5 of august then I can see if I,m still getting 16+ knots there. sounds like maybe I'm over proped ?

on a recent trip to lake powell (which is at 3700' elevation) we changed to a lower pitch prop in order to keep the rpm's up (even though top end speed was reduced).
then, when back at sea level, we go with our standard fare.

It sounds like you are overpropped. However, make sure you're trimming the motor up (and back down if necessary) until it achieves maximum rpm at WOT. There's a trade-off between the drag of hull angle versus motor trim that must be balanced.

Just my opinion, a Mac DF70 needs a 12-pitch at sea level. If it's a cleaver-clipped prop it might work at 13-pitch ... IIRC, Piranha sells a 14 x 13 cleaver. Regardless, at altitude you'll need to reduce pitch.

The 4000 rpms at WOT seems an "overpropped" signal, but another possibility is that you're not getting full throttle from the cable or linkage. It's really easy to pull the engine cover and see if you're getting full linkage throws.

You're fortunate to be heading back down for a sea level test. You should try to determine your speed/rpm curve at Puget, i.e. record each GPS speed with engine turning 3500, 4000, 4500, 5000 and WOT. (You need to be sure you're running at slack tide, or else you need two-way runs - a hassle. Then run same test with same prop in the mountains. Give both curves to any good prop shop and they'll make a pretty good stab at correcting the problem with a new prop.