This is from one of the new guys, so be patient.
I bought someone elses project (32' nordic os with straight hull) and am not sure of some of the hardware it came with.

I was told they are bravo I outdrives but the upper gearcase has imco cast into the housing. The lower appears to be much shorter than normal and I dont understand the reason for having that kind of setup. When I trim it out to the factory recommended max, 6 3/4 inch measured at the trim cylinder, It appears that some of the prop would actually be out of the water (on plane). The props are 4 blade Bravo I 28p. (That means nothing to me either) I saw the last dyno sheets on the motors (615 ci merlin) and they read about 700 hp.

The deciest owners son said it would go 85 gps and pull 4500 rpm. I saw the dyno sheets run up to 7000 rpm

My questions are: Are these bravo I or not?
What is the reason for the short lower?
Shouldn't this boat do better than 85?
Ill send some pics....Casey

You have a Bravo style drive. The drives are made by several manufacturers, Imco is one of them.

The shorter lower unit is there to raise the prop shaft thus reducing the amount of the lower unit in the water and hopefully increasing the top speed.

As far as trim is concerned don't be too concerned with the factory's recommendation. It won't apply if a shorter lower unit is installed anyway. At WOT trim the drives up, rpm and speed will increase. Continue to trim up until the speed starts to decrease as the rpm increases. Then trim down just a little and this should give you a good idea of where you need to trim for top end speed. As you become more familiar with the boat you will find where the sweet spat is for the drive trim at different water and speed conditions.

If you have the dyno sheets post your max torque and horsepower. 4500 rpm seems quite low for a high horsepower motor. If you have a 1.5 gear ratio in the drives it will take rpm in the low to mid 5000's to get in the low to mid 80's.

Matt

All good and accurate information.

You said your propellers are 28P. This means they are 28 pitch or have a theoretical forward travel capability of 28 inches in one 360 degree revolution.

I agree you will more than likely be looking to run in the mid 5000 rpm range. As far as better than 85 mph, I would need to know more about the boat. What is the wet weight? And what hp is actually being produced and at what rpm?

There are other issues that may arise as you get further into this project. Drive depth, prop pitch, prop rotation will all need to be addressed.

Use the props you have to determine a baseline. Be careful with the trimming since the boat is new to you and straight V's with shorty's can do funny things if over trimmed. No need to be scared just cautious.

Have fun!

OK, here are the power stats from the last dyno sheet. Im in Afghanistan right now and couldn't download the file.

RPM:5492
HP:685.8
TQ:655.8

Is the prop selection wrong?
Dry weight is 8500 but that was with the stock 454s. Fuel Cap is 205 gal
Casey

OK, then 4500 is definitely to low of rpm. I think you need to try the boat for yourself first. Second hand information on the current rpm can be just that...........second hand.

See what you come up with for a base line. I have a number of other ideas too, but we need some place to start.