Check my first post in this thread for a link to most of that already discussed and a little more detail on the project but the abbreviated version of your answer is:

Current set-up

OEM Mag 350 CID rated at 300 HP with a BIII outdrive. With a 2.2 ratio drive and 20P props the boat goes about 37 MPH at just under 5100 RPM's. Obviously this is a deep V style hull that will have higher slip numbers than the performance stuff you guys are accustomed to seeing (I think currently that works out to be close to 17%).

New set-up

Custom built 615 CID engine with an all new AFR head Im putting the finishing touches on right now that will make about 800 ft/lbs and probably 775 HP with a conservative cam thats friendly enough to introduce water in the exhaust to keep it down a few decibels.

Goals are low RPM cruise speeds of 45-50 MPH and a top speed of 65 - 70 MPH. With more power the boat should be pushing alot less water and I think the slip numbers will improve an appreciable amount. If so those goals seem reasonable.

Mr. Gadgets....Im not even considering a B-III.

From what Ive read and the people I have contacted it doesnt have a chance behind the torque output of this new engine especially when trying to push/accelerate a heavy boat which will load the drive even harder. Im likley going with a Konrad set-up.....just trying to figure which one would best suit my needs and lately Im leaning towards the new 560 version. I retain all the benefits of the DP, but now get the added strength I need and my new combo is exactly within Konrads 800/800 rating (which I sense is conservative on their end because they offer a warranty at those figures and guys have pushed them much harder than that in lighter performance boats). I still think with this much power and especially torque, a very aggressive five blade single prop would still be reasonably effective as far as low speed manuevering and getting on plane but it will never be as effective and as efficient as the right dual prop set-up from what I have learned so far. I would sacrifice a few MPH upstairs for the better holeshot and more efficient crusing speeds at lower RPM. Whether the boat goes 60, 65, or 70 at WOT it will be night and day faster and a bunch more fun than its current configuration.

My real focus lies not on top speed (although it will be fun to stretch her legs when the opportunity presents itself) but on much improved planing speeds so the boat feels 4000 lbs lighter, and I really like the fact I should be able to easily surpass my current WOT numbers while the new engine is just loafing. I would love to see 45+ MPH at 3-3500 RPM's. That would really extend the usefulness and range of the boat a whole bunch while probably using alot less fuel as a bonus.

-Tony

Tony, you are not going to like the way the boat leans over at cruise with a big single engine/drive/prop. And please take this constructively, I don't think you are going to see near that speed.

Spend some time here:

http://www.go-fast.com/boat_speed_predictions.htm

I will check out that site....what do you mean leans over? Are you talking about the attitude of the bow or side to side?

Trim tabs would handle any lateral issues as well as keep the bow down if necessary.....the power and the props with some trim adjustments should keep the bow up if need be.

Just want to qualify what your suggesting might be an issue and why....

Thanks for the input (good or bad).....thats why Im here....LOL

-Tony

PS.....I just used the calculater you linked me to (very good info) and with the larger slip numbers of a deep V, using the 225 factor that calls out, and inputing 9000 pounds with a few passengers to be real world, it spit out 67 MPH with 800 HP at the crank. I would be thrilled at 65+ and still pleased if it ran 60+.

PSS....Just played with my current known situation and reduced the "factor" by 25 to compensate for the poorer (higher percentage) slip figures I have now with more boat in the water (Brett says the boats slip figures will improve with more prop and more power being less of the boat will be in the water which makes perfect sense). So using 200 as a figure (simulating closer to 17% slip) and inputing my current 310 HP and the same 9000 pounds it spit out 37 MPH......pretty much nailed my current GPS top speed right on the money. Thats encouraging for my future plans as it lends more credibility to the potential or probability of reaching my goals. Even worse case scenario keeping the same crappy slip constant (which common sense tells you would improve with more than twice the power, better props, and less boat in the water) it still spit out exactly 60 MPH....the low side of my goals but one I would still be content with assuming planing times and low RPM cruise targets made big improvements as well.

Forget about the slip, not relevant in this calculation.

Do you have an accurate fully loaded for real not from a brochure (put it on a scale) weight on the boat?

Use that and your actual speed and your actual prop shaft HP (which should be 300 in this case) to calculate your constant. Then go from there.

I thought I saw 8000 lbs somewhere. That gives a constant of 191. At 9000 I get 203. Looking at mid to high 50's with 700 shaft HP.

That page will solve for any of the unknowns if you give it the other 3 that are accurate.

As to the leaning, you know the old every reaction has an equal & opposite reaction. Yes tabs will handle it, and also push the nose down. That is a lot of torque in a single.

OK....thanks for clarifying

The advertised weight of my boat (dry weight I presume) is 7200 lbs. I have a generator, AC, aftermarket stereo equipment, heavier engine (with the swap obviously), and was figuring on another 12-1300 lbs plus 500 lbs for a couple of passengers.

Honestly I really need to weigh the boat and when it comes out of the water I will do just that. I think 9000 lbs will cover it but it seems even an additional 5-600 lbs is only 2 MPH less top speed.

I used 9000 lbs and a 200 factor to get my current 37 MPH which is spot on to real world current performance based on GPS speed (speedo shows closer to 40). I pulled the 200 figure out of my butt however but I based it on the known slip I have know and how much the factor would need to change to refect that looking at how the other data was set-up. I laughed when it spit out 37 MPH so it seems the numbers are certainly reasonably close. All of this is theoretical in some respects but a big help in figuring out what tyoe of performance would be reasonable to expect.

I have to think the addition of close to 500 pounds feet of torque and about the same HP would be worth more than 20 MPH even in a hull as "wet" as mine but the only way to know is to actually attempt it which Im willing to do. Im just gathering as much intell as possible in an attempt to make more right decidions than wrong ones.

Do you agree that alot more power (and prop) will likely get more of the boat out of the water for improved slip and a better factor in your speed calculations?

-Tony

A few people have said what I'd say so far, like Mbam, I agree. Not to discourage a repower, but to consider how realistic it'll be and what your expectations are.

Not having done what you're considering, but something much milder, personally I wouldn't even consider trying to turn a high profile pocket cruiser into a go fast. Bigger motor to reduce the "sluggish" underpowered feel sure, but you'll find you'll get to a point, and it'll come quick, when the math no longer works and you're trying to push a tank through a wall with a tricycle. (I made that up)

So, my story, I have a 240 Searay Sundeck. Advertised 4900# dry, 65 gallons fuel, 20 gallons water and all the normal gear plas what we bring (like the 200# ice chest ) so we're pushing close to 6000# on the water. Original motor was a 7.4 MPI. They have plenty of torque for boats in the size range, made a good single power for smaller < 27'-28' cruisers but no top end to speak of which is perfect. Compared to the same boat with a 350 mag, it was much more responsive, quicker to plane and a little more top end simply because it can turn a larger 26p prop (2.0 gear) and it wasn't working as hard.

Fast forward to today. It's got a mild 540, mild in that it makes peak HP at 5300 (per PS dyno). I went to a cupped 30p BIII prop and now, I have about the best balance of performance that I can expect. At this point I'm against the hull, it just isn't made to run high speeds so mid 60's is it. In my case it's a very solid boat so at 65 MPH it's very stable, even in a little bit of rough water but with the huge wetted surface and the hull design itself, it's the 48000# gorilla I'm pushing at that point. And it's a deep v, 21* deadrise sport boat hull.

The BIII is still going strong at about 150 hours with the new power, but I'm easy on it. The best part is the enjoyment I get from the nice putt-putt cruise at 35-40 MPH. > 50-55 MPH is when it starts to have to work a little but when the same boat is working hard at 35-40, I'm waving as I go by at 50+. I spend most of the time between 2600 and 3200 RPM.

That said, I probably would not do it again simply because, it's over-kill for a boat like this and isn't necessary for 95% of our boating. I'd have a hard time giving up the torque of a BB, I admit, but this much isn't needed. I also have to admit, I enjoy having the power every now and then. It's a definite sleeper.

How often will you be running 65-80 MPH "if" you're able to get the power you're planning with a drive that will handle it?

How bad will the torque/list be?

How stable is that much boat with that deep hull, trimmed out and up on plane even if you can get a flat attitude?

How much torque/stress will the hull take before you start seeing problems/failures?

I think I'd save the $30k-40k you'll spend in the motor and drive (if not more) and repower with a 496 take-out and start planning a go fast, or start looking for a sport cruise designed for your boating style. Just my honest opinion.

Mine could easily make a lot more power but the goal was displacement to keep a pleasure boat a pleasure boat plus some.

I think what I did was crazy; I can't even imagine what you're considering.

Tony, again - forget about the slip. It is not related to mechanical efficiency. No, I do not think the additional power/speed will help. The drag factor is exponential. The additional speed is not helpful until the aerodynamic/hydrodynamic lift benefit offsets the drag increase. Not a factor at 60 mph.

I just hate to see someone go down this path. You are going to take a boat you enjoy now and create - ----

Im not talked out of my mad hatter plan just yet.....LOL

More info to share soon.....thanks for all your input!

-Tony

If I may be so frank, I think it goes way beyond Mad Hatter.

Since you cant seem to be talked out of it I say
just do it and take lots of pictures and keep us updated on the progress. Good Luck!