speed and prop operation theories

I was on go-fast.com today looking at the different types of stern drive units offered by mercruiser. I've noticed before in both real boats and the radio controlled hobby boats that I used to race that a certain design of speed boat uses deliberately cavitating props. What's the theory behind this? I grew up with bass boats, flat bottoms, and runabouts, so all I know is that when you catch some air on a prop, you slow down pretty quick. What could possibly be beneficial about it?

I'm looking at maybe using a stern drive in a Glen-L thunderbolt if I can maintain a good c/g. I was just thinking that a race-type drive might be lighter and smaller. But why the cavitating design?

(see my other post, "drive options; stern, vee, jet" for more on the project.)

 

I think you'll find these articles interesting, if not exactly answering your question or project:

http://www.tennantdesign.co.nz/articles/more.php?article=1

and

Surface-Piercing Propellers by Paul Kamen, N.A.

http://www.well.com/user/pk/SPAprofboat.html

 

Holy snot...

That's alot of good info. Those guys really got into the theory of it. It sounds like I'll stick to the "known" parameters for my first boat building project. I'm just now delving into strange things in my cars, after several "normal" projects. I would love to show up at the lake with a retro boat and the only surface drive those fisher-boys have ever seen, but I think that's way ahead of me at this point. I think I've only ever seen six jet drives, a dozen inboards, and one vee drive.

This will be the first non-outboard I've ever owned. I've been so concerned about "fishing space." For the first time, I'm actually realizing that speed is important to me, but I can fish out of a speed boat. Heck, all I need is room for three rods, one bait box, a net, and a tackle box. My wife doesn't fish anyway.

So, now my only questions are these: Of the stern drive offerings on the market, it seems as though the racing units have screws that are very shallow. I know shallow makes less drag, but it seems as though the screws are dangerously close to the surface. Whereas a normal cruiser stern drive puts the screw well below the surface and can still catch air, will a shallow screw make problems for me?

I know in my well tuned 15' tri hull with a 70 hp outboard, its pretty easy to get air to the prop if you trim it up for max speed. The cavitation plate (as I call it...is that the right term?) is installed at proper height. Will I have daily-driver problems with a race-type lower unit?

 

Which specific racing stern drives are you looking at? Is cost a consideration?

I've always wanted a 35' boat with #3 or #5 drives because their appearance really appeals to me but at the same time I find myself about 50k short I'm not really up to date as to what specialty drives might be available for a small boat such as your project though. And then you can always bridge the gap with aftermarket modifications like low water pickups, nosecones, standard drives mounted high or on standoffs with props designed to tolerate some air, etc., etc. Some of the other people on our forum like Jim who deal with race setups all the time might have a much better answer for you.

Considering your uses, maybe a closer look at jetdrives is in order though. No prop to snag lines (and compromise seals). Much better for shallow water than a stern drive. A cleaner transom, or at least a tiny bit more flexibility in some cases. And safer for watersports.

I personally like stern drives, but I also went to high school with a guy who fell out of his boat and wound up badly injuring his arm when he came in contact with the prop. And I mean a very severe injury. Stephen mentioned a similar story. So I've always felt a little overly-cautious about getting close to drives (and even feel a little strange being around them in the water, boarding by stepping on them (with the engines off of course).

 

I know you want to build your boat yourself, but as I was thinking about your project I though I should mention Alsberg , one of my favorite new boats, just in case you had not seen it yet. It's a 17' runabout with beautiful classic lines designed by Harry Schoell who is one of the most interesting and influential designers of our time (who has pioneered a number of innovations, but is most recognized for his delta conic hull/unique single step hull design) The Alsberg 17 is powered by a standard Mercury 175 hp sportjet.

http://www.alsbergboats.com/

 

prop dangers

I agree with you on prop dangers. I've been lucky, but I also treat them like a loaded gun... it can kill you, so safety is key. We go way overboard, like keeping the key out of the ignition when loading/unloading skiers, etc, but it can always happen. I don't think those safety concerns are as much of an issue for me.

I'd love the clearance offered by a jet drive, but this brings other issues:

1) Given my relatively low 270 hp and high top speed won't that put me in the range of least efficient use of a jet? (in general) Wont I be giving up major top end and fuel use?

2) I'm not sure I want to give up the docking maneuverability and high speed stability offered by an actual lower unit in the water.

3) Aren't they going to be nearly the same weight hanging off the transom?

You had asked.....

Cost is always a consideration, but if it means the difference between having what I want or settling for something else, a few bucks won't stand in my way. I was looking at Mercruiser's drives, namely these at this site: http://www.go-fast.com/mercury_racing_sterndrives.htm

The dry sump unit looks like it costs millions, but also looks like it only weighs half of an Alpha. They show three units on this page; one looks like it is totally wet, one looks like it includes some air at plane, and the dry sump unit as shown may include air, but can be spaced down up to 3".

 

I have to find the numbers, but unless I'm terribly mistaken these are heavier than the alphas because they're made to take a lot more horsepower. Plus you need a separate trans for some, and they really are many times more expensive. A #6 drive will run you around $20,000 and that only includes the drive not the transom assembly. A #5 drive runs around $12,000, and a #3A runs around $10,000. (These are ballpark prices - I haven't shopped around for a bit.) I don't think I've ever seen these big drives on a small boat like yours because there's really no need. An alpha drive should be fine for up to 50-60 mph, and after that a Bravo would be fine to 70-80 mph. (anyone else please jump in and correct me if I'm missing something... I'm starting to talk a little beyond my experience here...) How fast do you want to go?

 

Well, with a short waterline, slow deadrise, and 650 dry hull weight, I made a ball park guess of 1700 lbs for boat, motor, rigging, seats, fuel, battery, etc.

With that type of weight and 270 engine hp, (about 200 prop hp) I expect ball park of 80-90 mph as calculated by The Square Root of (Total Shaft Horsepower / Weight ) x Constant = Speed. I found that on the go-fast website. I'd be happy with 60-70, but I may as well go for it if I have it.

Help me think of what else I can move up front.

fire extinguisher, 10 lbs
battery, 30 lbs
ECM for the engine, 2 lbs
life preserver storage, 5 lbs
anchor, 10 lbs
I was thinking about using a windshield; that would add a wee bit up front.

That's 57 lbs that I can move about 10 feet forward from where it normally could be stowed in the transom area. I don't think that will offset the couple hundred pounds that a stern drive would place 6 feet to the aft, but I'll keep brainstorming. I think I'll contact Glen-L at this point to see if I can get more info on where the c/g should end up being in this design, then see If I can locate the weight to keep it decent. I'll include my weight in the model. I weigh about 240, so that will make a bigger difference than the average person, and I can't imagine every lending it to someone

Uh, yeah, that kind of cost will be prohibitive. What I need is a "light weight" stern drive. It seems as though (regardless of hp) I should be able to get away with a lighter drive with not so much boat to push. It seems as though the lighter boat could get away with less weight. I do this alot with cars. If I'm building a big piece of american iron with 400 hp, I know I'll need a Ford 9-inch rear with fabricated steel gussets, or maybe a really well-built 12-bolt. If that same engine is going into a late-model camaro, I know I can get away with a 8.5" 10-bolt which is three steps weaker. I know that either the lack of traction or the light weight of the car will tax the weaker axle less than a big road barge.

Thanks for all your help on these topics. If you ever have a car problem, or a boat powerplant quandry, let me know. That's what I'm good at doing.