Break out the Calculator!

[Ssor]

Hi Capt Chris, I am a rag bagger so I can't offer any advise on setting up the new engines. I can however tell you my observations of a great variety of power boats that pass me when I am sailing. Some are so nicely trimmed that their wake is practically flat when running very fast, others are running with their bow held high and plowing a furrow in the water that leaves larger waves than the tugs with a string of barges. I am sure that the latter burns a lot of fuel for no benefit. I believe that if the boat is designed to plane then it should slide on the water not plow a path through it.
I can assure you that I don't cuss the well trimmed boats that pass me in narrow channels,unless out of courtesy they slow to hull speed.

 

[Capt. Chris]

Ssor,
Thats the objective here , we want a well trimmed boat that handles well in a variety of seas. I already have a sportfish that weighs 24,000 lbs. , loves to drink fuel and throws a massive wake! (by the way It wasn't me in that narrow channel) This boat the goal will be to have a fast, fuel efficient planing vessel, if we could just get the balance of the major components close.
Thanks for your input.
Chris

 

[woodboat]

Adding weight to a power boat has less effect at high speed. Just read the thread on surface piecing drives. One poster had so much lift from the propeller that he could not add enough weight to make any difference at speed eventually abandoning the surface drive for tha boat. If the boat is close at rest then you need devices that are effective at speed. You need lift on the heavy parts not weight on the light parts

 

[Capt. Chris]

woodboat,
You have a good point, the props are another piece of the puzzle often overlooked. My good friend owns a prop shop and I consider him a guru. I never imagined that props correctly dialed in would be so effective. My 29' Topaz had unbalanced junk wheels when I got it, and the guru told me what the boat could do with reworked props. I was fortunate be cause he doesn't charge me much and really was into getting the balance of rpm,sog, and stern lift right. The result was 4 kt. increase and the stern lift was noticeable at speed. This project will use outboards, which I don't really any experience with, but prop many says he can work them also.

Chris

 

[woodboat]

Yup, Props can really change a boats trim as well as overall performance and top speed. I would imagine at the very least you would run some form of perfomance wheel in stainless?

 

[Capt. Chris]

woodboat,
The prop guy says big diameter with maybe a cup on the edge. I've got plenty of time to think this over, still finishing up the hull. Thanks for the input.

Chris

 

[woodboat]

I would use a cup as well. I had a single inboard on a 28 with a 14X11. It seemed a little small. there was a big performance boost going from 2900 RPM to 3300 RPM. I suspect prop slip. In theory the bigger prop will slip less but cause much more drag and reduce potential top speed. If it will be loaded down with a bunch of fishing gear than you may want more diameter. If it is lightly loaded, say one or two people with minimal gear than go smaller diameter bigger pitch. My guess is your prop guy is correct, you need grip and power over top speed.

 

[cyclops]

I was told straight surfaces are the most eff. and cupping is a loss of eff. to gain grip on a unstable prop size. Is either statement overall true ? His logic was based on any airfoil traveling thru a medium.

 

[woodboat]

Cupping

Many of today's propellers incorporate a cup at the trailing edge of the propeller blade. This curved lip on the propeller allows it to get a better "bite" on the water, resulting in reduced ventilation and slipping, and allows for quicker acceleration, or "hole shot," in many cases. A cupped propeller also works well in applications where the motor can be trimmed so that the propeller is near the surface of the water. The cup will also typically result in a higher top end speed.

First Site I googled

http://www.boatus.com/boattech/props.htm

 

[xstream]

topaz 28

I have a repowered Topaz 28 with twin yanmar 315 hp and she will do 34 kts max. , I would ke to know what you get with outboards. I sugest you put all the weight you can in the stern as the Topaz 28 is bow heavy as originally configured.

 

[Capt. Chris]

xstream,
I'll keep you posted, but you might have abit of a wait. We as still fairing out the hull. We added a bigger bow flare ,as you know shes a wet one. How does the hull handle at 34 kts.? I have had 3 of these topazs but the volvos would only push it 19-20 kts. We are shooting for about a 30 kt. cruise with 225s on the bracket.
thanks
Capt. Chris

 

[xstream]

Topaz 28 spray rails

I installed 10' spray rails on my 28 Topaz and it throws the spray out. I used a wedge shape from Hamilton Marine, they cut a 20' section in half & shiped it for a total cost of $150.
Good luck with your project, I am finishing a paint job with Imron.

 

[water addict]

Capt Chris,
Are you worried about the trim change from the new engine installation? I wouldn't worry too much about that up front. From your figures, you are removing almost 1000 lbs., or more than 10% of the original displacement, based on Topaz 28 figures I could find online. It's most likely the rise of the hull out of the water will offset the further aft center of gravity in the new engine condition.
My worry would be making sure the transom structure is designed to handle the loads from the engines cantilevered out behind the boat- wouldn't want to break off the transom!

 

[Capt. Chris]

water addict,
Thanks for the reply!. I have addressed the transom issue already. The boat was converted to a dive boat years ago by cutting the transom just above the waterline. Yeah , I know...why, right? Anyway, we went and laminated alternating sheets of 3/8 and 1/2" marine plywood using thickened epoxy. Built the thickness to a total of almost 3" after a layers of roving/mat on inside and 2 layers of biaxal on the outside. Oh, and also when replacing all the stringers,(we are gluttons for punishment!) I made large knee braces to stengthen the transom further. Without formal instruction on boatbuilding, I have adapted the motto of , if you don't know what the hell you are doing just add plenty of epoxy!!!

Chris