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Dan L

Dropped motor and RPM's increased?

9 posts in this topic

I have been experimenting with props and motor height. With a tempest plus prop I increrased my RPM's from 5800 to 6000 by lowering the motor from the middle hole on my Suzuki DF150 down one hole. I am one up off of the transom. I was surprised by this. I am thinking of lowering the motor to the bottom. Does anyone run their motor on the bottom hole. The boat is a crestliner fishhawk.

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Dan - That's strange. Generally (with a deep V style boat) you go up in RPM's the higher you raise the motor. The rule of thumb is to run as high as you can without "blowing out" while cornering. I'm not sure if this is a 100% rule but I have also found you can carry a SS prop higher than an aluminium one - probably because it is more efficiant and "grabs" the water better. Where is the bottom of the boat (not the keel) in relation to the cavitation plate on the motor? Unfortunately I don't have much to say about your original question because it is counter to EVERYTHING I have experienced. Paul

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Agreed... typically lowering the motor results in lower RPM's... not higher.

Did you change the prop and the motor height at the same time?

marine_man

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Are you sure we are talking the same "lowering the motor" manner? I.e. do you now have more empty holes above the bolts, or less? I have seen people say they "lowered the motor" but they really put the bolts through lower mount holes, meaning they actually raised the motor. Just something to consider, considering the higher rpms reported.

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Same prop at different heights. Definatly lowered the motor. I was just wondering about more bow lift since it seems to be a problem with this boat. I picked 1.5 MPH.

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Its possible your trimmed up/out a bit more then before you lowered the outboard, thus the higher RPMs. Also possible you were mounted too high and getting blow out before you could get the bow up, and the reason for 1.5 increase.

So the problem with bow lift was what, not enough or too much?

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It seems like any weight up front and I lose alot of speed. About 7 MPH between a light load and 2 guys and gear with front livewell full. If I fill the front 20 gallon livewell I drop 2 MPH. I now just use the rear livewell. Maybe that is normal be it seems like alot to me. I am just trying some props and different motor heights. I do think this boat has poor bow lift. I have also herd this from others on another site.

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Just to clarify, you say your one hole up off the transom. According to what your saying the distance between the motor bracket and transom before the move was greater. Correct?

As said that is generally the opposite but my above explanation could be why you increased your speed.

Where the mount holes are matters little. Those are drilled when the outboard is mounted. Whats important is you have the room to make the adjustments.

As for the loss of speed. Experiment with weight distribution and trim. Question, how much weight are you putting in the bow?

Trimming can overcome some of that extra bow weight but its going to take some off the top end.

One note when removing the mount bolts, you'll want to re-seal those to keep water from getting into the wood core of the transom.

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Correct. I run wihtout much weight in the bow. That and not filling the front livewell has helped. I might try lowering to the bottom hole.

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