yeehaw

I've got a '95 fountain 42 with staggered hp500's and bravos. I'm getting 72 at 5100rpm with unlabbed 4 blade bravo 28p props trimmed all the way out. It seems to me that trimming all the way would not be very efficient and I should get more speed with less trim. I just tried a set of mach stern drivers that are 26p and got about the same speed at 5100 rpm also trimmed all the way out. Let me know if I'm wrong here, but I think I should be trimmed about 4 on the gauge to get max speed. Do I need a prop with more bow lift or what?

Heres a pic running 72 - the trim level is obvious.

Coolerman

That is not a step bottom boat correct? And no, it shouldn't be trimmed out to the max.

throttleup

You are right. You shouldn't have to trim so high.

Looking at both the picture above and your slip indicates your drives are most likely too high. By lowering the drives, with spacers, you will find you do not have to trim so high. I wouldn't be surprised if you were to pick up speed on the top end as well as at cruise.

If you find that spacers are the solution you will also have to either change the props or do some work to them to allow the motors to run at the proper rpm. If you have any questions don't hesitate to call.

Matt

yeehaw

Thanks for the replies. It does have steps. I think 95 was the first year for steps on the 42. It also has a 16" notched transom. My prop shafts are 4" below the running surface.

I can get on plane pretty quick and with the bravos there is zero cavitation even in turns. At level trim I'm getting about 4800rpm at about 62ish. I start trimming out and get an additional 10mph.

Matt - so lowering the prop should create more overall lift? What do the guys do that install the short lower units? Do they have to trim all the way out?

bbladesprops

If your 4" below the bottom you should be fine. Dropping the drives may not give you the returns some think. You can definitely try it and see if the added leverage outweighs the added drag. Remember you have 42 feet of boat. I'm assuming the cabin is fully loaded. You are correct in saying that more trim angle decreases the drives efficiency maximum but, if you continue to gain speed with over trimming, than over trim. Your slip % is 18. I would like to see you try some 26P Maximus props. Your hull can benefit from a level running attitude if the slip goes down. The Maximus props will do this. Let us know if you wish to test some props.

throttleup

Yes, by lowering the drive you will increase the bow lift without all the trim.

Many who install 2" shorter lower units find themselves installing 1" spacers to get the boat to run right, it all depends on the boat and how it was set up prior to the shorter lower unit. If the drive was to low from the factory than the shorter lower unit will help. There are other reasons why one may want to raise the drive, motor changes, hull work, balance change, etc.

What many don't understand about spacers is how they can actually lower the drag of the lower unit. By having to trim as high as you do the lower unit is not moving through the water as it was intended to. It is going through the water at an angle instead of straight through. With a spacer you should not need as much trim which allows the lower to go through the water as it was intended to.

Also consider; all that water you are shooting into the air is wasted energy. That's lost speed.

Matt

yeehaw

Brett - The cabin is the normal layout w/berth, wraparound seating, small galley area, head/closet. It shouldn't weigh too much. What's the procedure for prop testing? How does it work and what is the charge?

Matt - that makes perfect sense, higher drag due to increased angle of the drive. What should I try 1" or 2" spacers?

Also, the props rotate inward. Seems like the props would try to suck the boat down rather than lift it. What do you think about changing them to outward rotation?

bbladesprops

Drive depth is a interesting issue. I remember years ago,when we first started to raise the drives. We saw these great improvement is speed due to reductions in lower unit drag. Raising the drives became one of the first options discussed when looking to gain speed. In and around 1999 and 2000, everybody and their brother was raising their drives. It worked on some applications but not all of them. Heck raising drives created new product offerings. Namely 5 blade props and spacers for bravo drives to get the props to hook up again. We had these items for #5 and #6 drives but really only for race applications. Over the last few years I have had more customers space their drives down to gain performance then raise them.

Lowering the drive can create more bow lift but only by using positive trim. Just lowering the drive creates drag. But, and it's a big but, if the arm length of the drive can now leverage the boat to decrease the wetted surface area, less hull drag wins and you go faster. So lowering the drive cannot reduce drag of the drive, it increases it. It can however reduce the drag of the hull. Which one will you experience. Testing will tell you.

Todays multi stepped hulls offer the decreased drag due to aerated water, therefore requiring less bow help.

Our testing process is easy and painless. Call us and we will arrange to send you the props you need for a very small processing fee. You keep the props a week or so and learn from them. No strings attached. Lowering the drives may ultimately help your situation. 5 blade props will also help at the current factory drive depth on your specific hull.

Outward rotation will not help your slip issue.

throttleup

I would definately try turning them out, as it will indeed change the lift of the boat. Possibly positively for you. There is no other way to find out other than to run the boat. I also recommend that you need to proceed with caution, when changing rotations as the boat may behae a bit differently.

Safe Testing.

Julie