May need sterndrive for a 400HP CAt 3406

[Willallison]

The only problem with surface drives is that they are only really efficient at higher speeds - generally over 40 knots or so - and tend to be pigs at slow to mid-range speeds. What sort of speeds do you anticipate running at?

Tad - have you ever come across a surface dive installation that was efficient throughout the speed range? If so, I'd love to hear about it - coz they're such sexy beasts!!

[Tad]

Winston,

Seems to me there were some pictures of your boat floating? From that you could make a guess at weight and required shaft angle. The success of any drive will rest with the propeller. A successful low-speed surface piercing prop. is a bit more exotic than you local shop can find. The other option is an inclined shaft in a tunnel or partial tunnel with full skeg for protection. That will kill performance, but may not be an issue? I would stay away from trying to build a trimming and steering (articulated as Arneson) drive, keep it simple.

Will,
The suggestion of surface drive was as a solution to very shallow drive in an existing boat with the given engine. Efficiency doesn’t really enter into the equation. Cost and reliability do. Thus I guess a $30k water jet is not an option. There are a couple of reasonable comments in the thread on low-speed surface drives in the Powerboats/propulsion section. Successful application of a low-speed surface drive will require specialized propeller design, but it certainly can be done. As you say, most boats with surface drives are optimized for something other than low speed operation. The pictures of the Levi's in the gallery seem to indicate most anyone could build one.


All the best, Tad

[Willallison]

There are a couple of reasonable comments in the thread on low-speed surface drives in the Powerboats/propulsion section. Successful application of a low-speed surface drive will require specialized propeller design, but it certainly can be done. As you say, most boats with surface drives are optimized for something other than low speed operation

Ah yes, perhaps I should have been clearer in my question: my take on surface drives is that they can only really be optimized for a relatively narrow speed range. In this they are no different from any other drive system of course; the difference being that with (most) other systems, the drop in efficiency outside the optimized range isn't so dramatic. Am I right? More specifically, have you (or anyone else for that matter) come across a surface drive installation that coul run efficiently throughout the speed range?

[AlaskaFisherman]

Originally Posted by Tad

Winston,

The major problem with outdrives is they limit prop. diameter, you have a big heavy boat that would really benefit from a large dia, propeller. The Konrads break down in commercial use, something you can't afford. A pair were tried here and were almost immediately replaced with Arnesons.

Through-the-transom "surface" drive would be the way to go, not necessarily Arneson. There is a long thread someplace in this forum on surface drives. Bluewater makes a big one, Pulse drive is another manufacturer, Simplicity, Tri-Max, and Levi are also suppliers. But you are fairly handy, you can build your own, have a look at the Levi, they do work and are real simple. There are pictures of them in the gallery and see the drive sources in the Directory.


Tad

Tad, I like the looks of the Levi. What do you know about their pulling power?
Winston

[AlaskaFisherman]

Since the hull is looking a little flat at this stage, I was hoping of going with a design that would give high speeds. For this boat that would be 30 mph.
Actually, 20 mph would be fast for our fishery.
There are several boats with outdrives but they are not quite as heavy as my boat but they are not as wide either.
My first design was to lower engine two feet so shaft and prop are parallel with hull, thinking this would give more speed. I would taper hull to prop from engine for smooth water flow to prop. I hear 7 to 8 % of power is lost with the
7 deg. angle the prop has now. Will the drag from lowering the engine/reduction gear into a 2 foot wide chamber below the main hull help or hurt speed?
The boat might be around 20k lbs dry after I add some more length, might as-well figure 25klbs with a few fish on board. She is 16 feet wide and I plan to finish her length off around 44 to 46 feet. She is 35 ft. now.

[Deering]

Well, you might not win any Most Graceful Yacht awards, but I like it!

Seems that, depending what's inside, you could build a tunnel into the bottom of that hull. Use the same prop/shaft/rudder setup, only it'd be 16" higher.

[AlaskaFisherman]

Originally Posted by Deering

Well, you might not win any Most Graceful Yacht awards, but I like it!

Seems that, depending what's inside, you could build a tunnel into the bottom of that hull. Use the same prop/shaft/rudder setup, only it'd be 16" higher.

That is what I had in mind. Any thought as to effects on speed?
This stern is not the finial plan by -the-way, its just allows me to fish her inbetween cutting and welding.

[Deering]

Not sure...

You'll lose some lift due to reduced planing surface area. That will hurt.

The tunnel will probably contribute some drag due to the high speed water running along the walls. That will hurt.

You'll be able to get less shaft angle. That should help.

The big unknown in my book is the prop efficiency in the tunnel. I think the devil's in the details of the tunnel - how long, how close to prop tips, entrance detail into tunnel, etc.

If you want to spend a few bucks getting the answer right, you might touch base with a firm like Hydrocomp (www.hydrocompinc.com) to help model it. Don Macpherson is the technical director and occasional list member here. Might be worth it before you start cutting & welding. Don has quoted me reasonable numbers for similar services.

[AlaskaFisherman]

Originally Posted by Deering

Not sure...

You'll lose some lift due to reduced planing surface area. That will hurt.

The tunnel will probably contribute some drag due to the high speed water running along the walls. That will hurt.

You'll be able to get less shaft angle. That should help.

The big unknown in my book is the prop efficiency in the tunnel. I think the devil's in the details of the tunnel - how long, how close to prop tips, entrance detail into tunnel, etc.

If you want to spend a few bucks getting the answer right, you might touch base with a firm like Hydrocomp (www.hydrocompinc.com) to help model it. Don Macpherson is the technical director and occasional list member here. Might be worth it before you start cutting & welding. Don has quoted me reasonable numbers for similar services.

I might well do that, it is looking like 2007 before I make that cut so I have some time.

[AlaskaFisherman]

Originally Posted by Steve H

I would contact Konrad. They build a very hefty I/O that may work. Go to Offshoreonly.com and do a search. If any one can break a drive the guys on that site can.

Steve H

Steve, I am going back to try these two listings. Out drives may be my only hope of increased speed. Thanks for the input.

[AlaskaFisherman]

Originally Posted by AlaskaFisherman

Tad, I like the looks of the Levi. What do you know about their pulling power?
Winston

I am thinking of figuring on 25 klbs after adding 10 more feet to length and a few fish in the hold.

[longliner45]

we had twin vovlo210s with duoprop out drives on 31and35 jc boats with 6000 lb loads we could do 20 knots .but no matter what drives them they will have a top speed of about 40 knots .you can put tractor wheels or speed wheels on them in 2 minutes ,for more info go to longliner45