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Hi Guys,

I'm from Germany and I'm rebuilding/customizing a 1969 Katama bought from Italy.
I bought a 1989er Mercury 90HP outboard with a Quicksilver stainless steel Prop. On the prop is 48 16990 A40 20P witten.
Can someone tell me if that prop is ok for that boat engine combo? And how high should I mount the engine?
I'm using the boat for fishing. The load can be two or three persons, a dog and lots of fishing gear :-)

Tank you in advance and greetings from Bavaria Germany

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No expert but I have a similar hull to yours and a 90hp 2-stroke Yamaha motor of similar age to your Mercury.
My prop is a Yamaha painted stainless 17-K which is 13" diameter and 17 pitch.

I think your prop is 13 1/4" diameter and is definitely 20 pitch.

That seems like it will not allow the motor to reach max rpm's with a normal load. You can test and see if possible.

My motor is mounted 2 holes up [bolt goes through the third hole from top]. Yamaha has 4 holes and your Mercury may have 5.

I raised the motor those 2 holes several years ago and am very happy with the result.

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Thanks for your help!
Anyone else here with a similar or the same engine running on this hull?
Regarding the hight I have to check because I can only drill the upper balck and the lower green holes. So I think I can not use all holes from the engine.
Kind Ragards Christoph

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Older hulls have a smaller splashwell so your options on height may be limited or require drilling new holes or using a jackplate. Yoou can mount as high as you feel is right for your setup, water conditions, etc.

A 20P is probably to much prop but you won't know until you mount it and see what RPMs you are getting. That also will depend on the engine height. The higher you go the more pitch you usually need. You will need a tachometer to find out what RPMs you are at WOT trimmed all the way up/out.

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Tanks for the advice.
So you would reccomend to hang the engine as high as possible?
If I drill the green holes I can mount the engine lowest in third hole from top holes and first hole from lower holes. (Mercury 5 holes in total).
This old Mercury has 6 1/2" betwen the upper 3rd hole and the lower first hole. You know what I mean?
That is quite high right? Do you think thats too high?

Regards Christoph

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Now it should be easier to understand what I mean.

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So you have just the black holes drilled? If so, yes if you drill the green holes(if you have room) you can raise to the red which is not too high.

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Sorry I wanted to post a picture but it did not work....

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try again

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and the measurement

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The lowest holes I can drill are the green ones. Otherwise I get out of the splashwell and I can not bolt throu hull.
As I´m right the anticavitation plate is the same hight as the hull when the engine is mounted in the first hole at the upper holes right?
I think I can not go to red holes. If I do that I´m in the fifth hole at the upper holes. That means in my opinion that I raised the motor 4 holes right?

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Drill the green as that still allows you to through bolt. It should allow you to install the bolt in the third hole from the top.

I still think your prop is too much pitch to get best performance, but you can check that when you run the boat.

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Yup

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@ mtown
Ok than I will drill the green holes and mount the enigine in the third hole from top.
But than I can not lower the engine. Do you think I will maybe need to lower it or is the third hole from top the minimum hight anyway?
Regarding the Prop, I know it might be too big but I can try. What would be the perfect size prop? Or do I have to measure WOT RPM first?

Regards Chris

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I don't think you will need to lower it but if so you just use the same hole in the hull and drop to the next higher hole on the motor.

I am no prop expert. My 90 hp Yamaha has a 17K painted stainless steel prop which is great for my motor. It is 13" diameter and 17 pitch to the best of my knowledge. Your gearcase may be different and I would get advise from a real prop expert before buying a new one.

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You need to try it first and see. You also need to know WOT RPM max and trim it to its max when testing. Rule of thumb is 200rpm for every inch of pitch.

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Christoph,

Send me an e-mail. I may be able to point you toward some educated advise on the prop.