How fast can I go
 

Hello :)

I have a 21 foot prowler boat by Cooper Yachts that I'm going to repower and I'm curious as to what you guys think about the hull design...

Right now it powered by a 4.3 mercruiser alpha one and I'm building a 383 for it.

Right now it does 45 and my goal is 55+

I don't know how much it weighs and I don't know what the beam is... So I'm hoping you could take a look at the pictures and hazard a guess :)

I'm thinking it should do 60 without running into chine walk but I'm no expert... To get me close I figure I need another 150 hp but maybe not?

Opinions?

Thanxs

Ps I'm aware of the alpha one hp limitations... Going for a re=gear or maybe a sei drive... And if I can't keep the drive from blowing up I guess a bravo.. Just hear a lot of people saying I might be able to make the alpha survive if I'm nice to it lol




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I assume you're down at 225 horse or 190 horse with the 4.3. If it's 225, I'm guessing you'll need 130 or so HP to add 10 mph. A 383 could easily be built to about 400 HP. So, I think you'll make it to 55 and then some. At 400 horse, still with the Alpha if you're kind to it, I could see 60+.

I DON'T know the boat and whether doing that makes sense or will be within the strength margins for the layup, nor whether it the speed will work with the hull shape, etc. Guessing it's fine if things are strong enough.

Screw that SBC. Call pslonaker and get one of his super awesome custom "Shootah Bullet 540s" built in has magic room and really make that bayliner lookalike fly:picard1:

Thanks for the response...

The boat is mint... And I got it for the right price. Trading it in and buying a boat that does what I want is going to cost more than the motor and I'll still be with a boat with an old motor... Most of the boats around here in the 20 thousand dollars range (Vancouver Canada) are stock (meaning 50mph) or rotten... I don't think I can sell the wife on a 30 thousand dollar boat right now lol.

Like I said it's mint... Never seen a winter outside and always in a heated shop for the winter. It's well built... definitely not a bayliner. Cooper Yachts only built these small boats for a handful of years so there's not much information about them... Other than there bigger boats are well regarded.

I really didn't want to buy the boat when I saw it but couldn't pass on a boat in the kind of shape it is in for the price point... It was perfect except it's to slow. Now that I have 4 seasons on it I can justify spending some money...it hasn't cost me a dime lol.

My worry is if the hull can do 55+. Building hp isn't cheep and like it's been said a 383 can be built for way more than the boat could reasonably handle.

It looks to have a small pad and lifting streaks to the transom. They should help to keep the back of the boat from getting to loose on you. I don't see any trim tabs, might want to put some on it to add some stability. I think 55 is obtainable, maybe 60.

Thanks... That's the opinion I was looking for...

I know it's not an offshore boat but thought you guys might have an opinion on the hull...

I'll definitely look into trim tabs...

What length of tabs and where would you place them in relation to the keel?

Read somewhere about smart trim tabs... Any good?

I wouldn't go crazy with expensive trim tabs, you would probably be fine with a Bennett trim tab kit. Your going to want to put them on the outer edges of the transom. You can mount them to match the V angle or have them mounted straight. It's kinda preference, some say if you angle the to match the V that they will cause the back of the boat to walk in one direction or another. There was a post a while back on this subject you can do a search and find it. I don't know anything about automatic trim tabs. Me personally I like to control them my self to be able to compensate for different variables. So no I wouldn't go with the automatic trim tabs.

Does it porpoise now ?

if you have the space, fog, winterize, and store that 4.3 so when it's time to move on, the stroker can stay with you.

Not really... But I always felt like I had to trim it way down in rough water to get it to ride good... Calm water and I can trim it way up...

That there is a really great idea... Never thought about that! And it's not like it's worth a ton of money so yeah great idea

Is there room lengthwise?

Make sure you take that Bimini top off before you get it all revved up. Hah

There is... A v8 was an option for the boat

ok... :)

That calculator doesnt really account for slip so its a little optimistic but close.

What prop are you running now?

Also idk how light that boat is, but I had a maxum that went over 60. It didn't chine walk bad and handled it pretty well. The only problem I found is when going that fast and hit much of a wave it would get air under the nose and start picking it up. It would lift enough you had to back off the throttle slowly or it would flip. 20 foot open bow boats are usually pretty light so just be careful to get to know it when it's done.

I'm not sure... I know it's a hi-five. I top out at 45 at about 46 47 rpm

The more I read and look at other boats I think I should be able to get to 60 without much problem. I've operated a couple of boats that were over powered an 60 plus... Never had a problem with wanting to blow over but chine walk was an issue.

Putting the parts list together for my motor... Looking at proly 350+ hp.

Should get me to almost 60 I think... Adding 150hp should be a big Improvement lol.

Yup, if a 350 hp 5.7L won't get that to 60,it's a pig. LOL.

My thoughts exactly... And I'm building a 383 with the added torque... 350/360hp should get me to 55 and maybe 60. I'm not really interested in the top speed... But really looking forward to cruising at 40 without the motor working that hard

A High 5 prop probably isn't the best prop for that, especially for top speed. Look for a Mirage Plus style prop to at least try with your new setup. Regarding how it will handle at top speed depends on the hull, conditions, prop, driver, and tabs.

That boat will do 60 all day with a 383 unless you totally wimp out on the engine build stuff. My 4000 lb 23 degree load does just about 60 with a fairly well designed 383.

From the pic, that drive/ X dim is in the basement. That will make it quick to plane, but slower at WOT.

speed cost $$$$ how fast do you want to go...not a performance boat, so ?

Most 21 foot scarabs ran 60 with 260 horse and a 1.5-1 drive.

Your boat has a pad bottom like the scarab so 55-60 with a mild 383 should be cake unless the bottom has a big hook in it.

What is the biggest power package that they installed OEM on this hull?
You cannot just throw more HP at any hull, and expect it to improve speed and act/handle accordingly, linearly.
Sometimes you reach the limit of a hull's capability, and when you do; it let's you know in a hurry, and usually it's not pretty.

Thanks for the response... I thought the same thing... Not much I can do about it though :( I can live with the lower top speed a bit... But what do you think it would cost in mph? Say at 60?

I want 60... But not to drive around all day at top speed... Just need a nice cruising speed of 35 to 40 without working the motor hard. I have a 50 minute drive to our cabin at 30mph... B nice to do it in 30 minutes lol.

Imho 60 isn't really performance boat speed and I agree it's not a performance boat... But that doesn't mean it can't be fun to drive :)

That's what I want to hear! :D

But what do you mean by "big hook"?

A 5.7 was an option...

I agree that you can't just throw hp at it without thought... That's why I'm here asking for your guys opinion on the hull design.

I'm well aware of the difficulties operating a overpowered boat. I can live with moderate chine walk. I don't spend allot of time driving my boat at wot. With the opinions I've gotten so far and looking at performance boat hulls I think it should do 60 without to much trouble. I am going to add trim tabs... Hopefully they will help

Ya know it's funny...

When I was first started thinking about re-powering I thought about trying to get another 50hp and 5mph out of the 4.3 (almost impossible I might add)

Then it was 100 hp and 10 mph from a 5.7...

Now I'm at adding 150 hp from a 383 stroker and hopefully 15 mph...

This is addicting haaaha :D

If you have a .43 with a 4 barrel @205 hp you can achieve 55 mph with a 23p( ballistic) prop and some fine tuning.
If your going to beef up the 383 you might want to invest into a Brovo outdrive . Your virtually going to double the HP.
The alpha outdrive will not last long.

I agree... But I'm shooting for another 150 crankshaft hp which should put me at 320-330 prop hp and not quite double the hp... Keep hearing that it will survive if treated nicely and serviced regularly..

Every time I read this title I ask myself, "How much money do you have?" Lol.

I have no practical experience on the matter, but the folks getting good results with bigger power on a stock drive attribute it to lots and lots of cooling water over the drive. Spikes of jumping waves and hard holeshots are brutal, but a lot of failures happen in flat water with long runs. It's just a constant load with no relief.

I agree... Drive shower and I'm thinking sei drive (re-gearing will almost cost more)

with my build I'm shooting for 350 crankshaft hp... That should put me just over 300 prop hp. Alpha one is rated at 300 max prop hp so although I'm adding allot of torque with the stroker and proly 20-30 hp... I'm feeling pretty confident that it'll survive lol

My budget is 7 grand...and I don't have much money... Can I borrow some?

:D

If your worried about breaking an Alfalfa with a 350hp 383,build a 350hp 350.