Newb looking for a crash course in performance for a scarab sprint

[chevyracer5613]

Hey there, I hope I am posting this in the correct section. I am not new to boats, nor am i new to performance, I have been racing since I was 8 years old and naturally because of that always been a performance enthusiast. However I am new to modifying boats for additional performance.

I am helping a buddy of mine out, he has a early 90's scarab sprint with the 3.0L 4 cylinder motor and a SS rapture prop stamped 9513 and 1425r23. I am not sure if the speedo is accurate but it claims to be running about 50 mph at about 5000 rpms (45 with three people in it). I am wanting to make this boat faster without spending loads of money, from what I have read if this boat is actually going that fast then the little 4 banger is allegedly being pushed to its max already? As you can imagine the low end power on the boat is pretty poor, as would everyone else i'd love to get more power all around and looking for some opinions.

From what I read here are a few things I am considering:
1. xs smart tab trim tabs
2.experimenting with different props
3.checking and maybe advancing the timing 2*
4. possibly removing the stock flame arrestor and installing a K&N air filter/flame arrestor
5.some naca ducts to get heat out of the engine compartment
6. perhaps motor swaps? 4.3, 5.0, 5.7? Not sure of the complexity, price, or gains to be had with that
7.perhaps hotter plugs, or a bigger coil, maybe MSD???

Also on a side note, we were jumping some wake the other day and when we landed we heard a exhaust leak, where the first 90 degree elbow is for the exhaust out of the manifold cracked.

Lastly I'd love the hear some simple theories on possible experimental ideas for speed. For instance, I had considered a spoiler (or rather a lip) to help induce lift of the front and center of the boat. I assume that might be helpful because if less of the boat were in the water perhaps there would be less drag?

Anyhow it's late and I could ramble on forever, I am open to any and all suggestions!

 

[Bondo]

Re: Newb looking for a crash course in performance for a scarab sprint

I am wanting to make this boat faster without spending loads of money,

Ayuh,... Welcome Aboard,....

You can try #s 1, 'n 2,....
#3 will get you holes in the pistons or run-on caused reversion...
#4 is a waste of money, but hey, it's your money, your Buddy will love you for No performance gains...
#5 It's already well vented,....
#6 you'll need to swap the entire driveline, Pulleys to Prop...
#7 is Burnt pistons, or possibly an Explosion...

Btw,... Fix the exhaust, Now, not later....

 

[chevyracer5613]

Re: Newb looking for a crash course in performance for a scarab sprint

Bond-O, Thanks for your response. Correct me if I am wrong but unless boat motors are tuned right to the edge there should be some room left to adjust timing.........as long as its not detonating/pinging I dont see why it would hurt the pistons...care to enlighten me? I had also planned on running octane booster through it as well to help prevent detonation.

Is the motor swap worth the hassle?

I had planned on fixing the exhaust, and was actually hoping to upgrade while at it but from what I read on those the system already flow pretty well and I wouldnt notice any gains unless I messed with the manifold also?

thanks again for your time!

 

[DuckHunterJon]

Re: Newb looking for a crash course in performance for a scarab sprint

I'd say if you are hitting 50 (gps?), you are getting alot out of that motor as it is. In my 17' Glasstream with the 3.0 I couldn't do better than 42 with an empty load. I'm sure you could do lots of little things to the motor to wring out another few hp, but I doubt you are going to get more than 5 - 10% out of it for a lot of investment. Boats aren't like cars. Bump a car motor an additional 10%, you'll get almost a 10% increase in performance (yeah, not exactly, but close). Bump a boat motor by the same amount and you will see little difference. Water drag is alot more than wind resistance.

My suggestion would be if you really want speed, upgrade boats. Look at the boats that are running from 50 - 60 - 70 mph. There are BIG differences in HP.

Just my $0.02.

 

[H20Rat]

Re: Newb looking for a crash course in performance for a scarab sprint

Is the motor swap worth the hassle?

More headache than you can imagine, at least to go to any sizeable jump in performance. If you were going from a v8 to a slightly higher hp v8, it would be easy. In your case, you are going from a inline 4 to either a v6 or v8. The mounting would all be different, none of the accessories would be reusable. The biggest problem would be one you might not have even mentioned. Your drive most likely wouldn't be usable either. Its been sized for a lower power 4 cylinder, and if you did manage to couple it to the new engine, it might destroy it in short order anyway.


I'd never take a GPS near it, and be happy with the speedo! (I highly doubt it is actually hitting that on gps, but best to not confirm and just think it is... boats are for enjoyment!)

 

[Philster]

Re: Newb looking for a crash course in performance for a scarab sprint

If you want another 2-3 MPH from engine performance goodies, I think you need another 5 grand.

Boats vs cars: Boat top speed is hard to affect with performance hop ups, because we tend to forget that in the automobile world, we seek acceleration times. It is very hard to boost performance where the engine is already doing a good job with its peak power: 4500 RPM plus.

Think about cars: Acceleration times come at the expense of top speed (say jumping to 4.11 rear), or an aggressive cam that gets the engine to light up fast. People don't drive around in cars wondering how to make 'em go 148 instead of 145. Performance goodies make you save a few tenths of acceleration.

For a boat, you'd be hard pressed to do anything beyond making sure the engine is in tip top shape, invest in maintenance (man does this matter on a boat. See your exhaust comment. Bad boat exhaust = bye bye engine sometimes!!) and make sure the prop is modern and tuned to the exact specs of the boat/engine/use!

You're splitting hairs... and they only hair splitting that will make any kind of difference in a 4 cyl boat running plus-50 MPH is a perfectly tuned prop. It might deliver the same or very slightly better top end, but be able to hook up, ventilate less and just provide all around better feel and response.

I love watching how people hop up their boats looking for more MPH. I've seen some big HP jumps = 0 to 2 MPH. Hard to swallow 2500 bucks worth of work for 1 MPH and reduced drivability. Some guys destroy the power curve and LOSE 2-3 MPH.

Make sure she is a happy, maintained engine, and run a modern, cupped and (dare I say) labbed prop if you are THAT serious. Props are general use, but they don't do everything they can for your exact boat until a prop shop works with you/it to make it spot on.

Lot of work/$$ for little return in most cases.

 

[45Auto]

Re: Newb looking for a crash course in performance for a scarab sprint

There's only two ways to make your boat faster: Increase power or decrease drag.

A very good rule of thumb for boats in the 50-60 MPH range is that it takes 10 HP to go 1 MPH faster.

So to pick up any kind of meaningful speed, say 4 or 5 MPH, you would need to increase your engine output 40 to 50 HP. Gaining 40 or 50 HP from a 140 HP engine while keeping the RPM in the range where the outdrive is happy (around 5000 RPM) is going to require some kind of forced induction. I can't count the number of posters on here who were going to turbocharge or supercharge their 3.0. None of them have ever posted back.

You can fiddle with things like air cleaners, props, timing, tabs, etc. You MAY see 1 or 2 MPH difference total if you're lucky. Of course, you could also probably see 1 or 2 MPH difference just due to atmospheric or water conditions, less beer in the cooler, etc.

The other way to go faster is to decrease drag. The spoilers you talked about on the boat won't do anything for you. It would take something sized along the line of a Cessna 150 wing to do you any good at 50 MPH, and the pure ugliness of the thing would slow you down more than you would gain from any lift it provided .....

99.99% of the drag slowing your boat down is coming from your hull and drive dragging in the water. That's why serious racing boats are "tunnel boats", or hydrofoils. As much of the weight as possible is supported by air instead of water, and the only thing touching the water is half the prop.

There's not much you can do with your basic hull form, some hulls are faster than others. I believe that some of the Stingrays with the 140 3.0L will do about 60 MPH. If you're really serious about going faster, build something like these bolt-on hydrofoils from the 50's & 60's for your boat:

http://www.histarmar.com.ar/InfGral/Hidroalas/The Up-Right Hydrofoil Kits.htm

Nothing wrong with piddling around with the boat if that's what you enjoy. Just don't be surprised when you don't see any significant gains. Most effective way to go faster would be to sell the 3.0L and buy a boat with a 5.7L in it. I believe that WCA_Tim (on these boards) has his 17', 5.7L bowrider up to almost 90 MPH. If I remember right he was making somewhere between 500 and 550 HP.

 

[Philster]

Re: Newb looking for a crash course in performance for a scarab sprint

T
You can fiddle with things like air cleaners, props, timing, tabs, etc. You MAY see 1 or 2 MPH difference total if you're lucky. Of course, you could also probably see 1 or 2 MPH difference just due to atmospheric or water conditions, less beer in the cooler, etc.

.

There you have it. Spend all your money on fancy air cleaners and MSD ignitions and you won't be able to keep the beer cooler stocked = FASTER BOAT!

 

[chevyracer5613]

Re: Newb looking for a crash course in performance for a scarab sprint

DuckHunterJon- You are completely accurate on the fact that water produces more drag then air, I learned that very well in my fluids class (I am a mechanical engineering student). Aside from that fact I had hoped boats might be more influential then they are considering they might have less drivetrain losses then a car and therefore a higher percentage of the gained power could be used. Perhaps theres alot more to it then that though haha.

Smokingcrater-I had assumed and feared everybit of what you mentioned, and its for that fact that if I were going to do anything as far as displacement goes i'd be more tempted to yank that engine and bore it some before swapping motors....but again I might run into the problem of the drive being to weak. I also like your idea of not putting a GPS in there, the boat is already out running most of the other boats on the lake...it'd be a shame to find out we are alot slower then we think haha.

Philster- Generally speaking you are right that most people seek acceleration over top end in cars, however I actually go for both since I am a circle track racer and unfortunately that ports over to everything I do haha. Also I agree that a 2 mph gain would not be worth a $5k investment, however if I could keep the top speed around the same and significantly improve acceleration I think my buddy might be completely satisfied with his boat. Speaking of this compromise and props I have always been curious about a variable position/pitch prop (one where the fins rotate to change pitch, not a fixed blade) in theory it would seem like that might provide a significant improvement in bottem end power and maybe a very slight change in top end, however most of these props seem to come in at a price of $1500-2000 new (about half that used).

Speaking in theory, would it not help to have some aero support to lift the boat while it was at speed to help reduce the drag by having less boat in the waterr,isnt that the point behind hydrofoils?

Also the bottom of his boat is not painted yet, I was curious if there were any good suggestions there for speed....I say this because a slightly roughened surface will flow fluids over it quicker because it "sheds" the sticky fluid, same principle why a golf ball is dimpled.

Lastly I have read many places that these 3.0 L 4 cylinder motors are stressed out enough and adding more power will surely blow it up......but what is actually the weak point in these motors? Also I have read contradicting posts online, some claim the pistons are forged and others say they are not...can any shed light on this?

Again thank you all for your time and comments, I really appreciate them!
David

 

[Bondo]

Re: Newb looking for a crash course in performance for a scarab sprint

and its for that fact that if I were going to do anything as far as displacement goes i'd be more tempted to yank that engine and bore it some before swapping motors...

Ayuh,... An Overbore don't make anymore Power as the displacement difference is Tiny,...
It's just for refreshing a motor...

would it not help to have some aero support to lift the boat while it was at speed to help reduce the drag by having less boat in the waterr,
I say this because a slightly roughened surface will flow fluids over it quicker because it "sheds" the sticky fluid, same principle why a golf ball is dimpled.

You're still mixing Fluid dynamics with Aerodynamics,...
They're Very Different...

Also I have read contradicting posts online, some claim the pistons are forged and others say they are not...can any shed light on this?

They're Cast, or at Best, Hyper-'s....

Tell your Buddy to Enjoy what he's got, before you Blow it Up...

 

[chevyracer5613]

Re: Newb looking for a crash course in performance for a scarab sprint

45auto- I had actually strongly considered forced induction, I am very familiar with turbos however I had considered a roots style supercharger just because the carb could be mounted ontop of that instead of having to find a way to make that carb boost ready?(unless those carbs are able to take boost and meter fuel correctly which I doubt).......perhaps there a boost ready marine carbs that I could bolt up to it? Even if so then i'd be curious about the strength of the drive. As you mentioned hydrofoils have crossed my mind a few times although I have been curious on the stability you might loose from them? I also agree that a spoiler needed to create lift of that magnitude would probably have to be of decent size and definately hurt the aesthetics of that boat haha.

Back to another crazy theory, do you think anyone has experimented with injecting a pocket/s of air (bubbles) around the bottom in order to reduce drag? I ask because torpedoes use this to increase there speeds significantly.

 

[Bondo]

Re: Newb looking for a crash course in performance for a scarab sprint

Back to another crazy theory, do you think anyone has experimented with injecting a pocket/s of air (bubbles) around the bottom in order to reduce drag?

Ayuh,... Google "Stepped Hulls"...

 

[rbh]

Re: Newb looking for a crash course in performance for a scarab sprint

I guess, if you could figure out how "not to suck in the water", you could vent you exhaust out behind/below the bow.
The faster your motor spins the more exhaust you pump out, and up you go on a bed of exhaust.??????

 

[Philster]

Re: Newb looking for a crash course in performance for a scarab sprint

Variable position prop would help acceleration. The top end is what it is, if the prop is biting and the engine is in the max operating RPM range.

Side note to all this: If you had a 40-43 MPH boat, with an ancient prop, turning 4300 RPMs at WOT, and some mechanical issues, you'd get a slew of responses on how to get the last few hundred RPM, how to get the right-sized, modern prop with good cupping for bite, and how to get things like the trim set up just right so that she can stretch out to 4600+ RPM and get those missing MPHs. We'd be able to make proper, meaningful and practical suggestions to get all the motor has to give.

The problem is that you've stumbled in here with a 4 cyl boat doing 50+ MPH. You are officially at the point of impracticality. You're at the wall for that setup, give or take a tweak or two.

Get a modern, 4-bladed stainless prop that keeps you in the max RPM range, enjoy the improvement to the bottom and mid-range, enjoy more bite for acceleration and less ventilation, and maybe kiss away a 1-2 loss in top speed, and call it a day.

 

[chevyracer5613]

Re: Newb looking for a crash course in performance for a scarab sprint

Bond-o-From what I have learned thus far the principles between aero and fluids are identical, they are both fluids and are bound by similar equations since air is a fluid (from a engineering standpoint). If he became desperate enough for speed and keeping that motor could I not just replace the pistons with some forged pistons and maybe even up the compression a little? With forged pistons, although detonation is still bad, it wouldnt kill the motor as quickly, and i'd think as long as I compensated with the correct fuel and made sure to keep EGT's intact that the motor would be fine (same concept of a street motor vs. a race motor)......but again I might run into drive issues, does anyone know the strength of the drive units on this style boat? Also thanks for directing me towards the stepped hull design, thats identical to what I was talking about! I wonder if there have been experiments with a wicker bill/gurney flap to help reduce drag?

It seems to me like the quest for significant MPH gains on this boat will be difficult without alot of money, however would the goal of better overall performance be more obtainable? (increasing bottem power without hurting top end much) Is my only option for this going to be changing out the prop or getting a different boat haha?

 

[Philster]

Re: Newb looking for a crash course in performance for a scarab sprint

It seems to me like the quest for significant MPH gains on this boat will be difficult without alot of money, however would the goal of better overall performance be more obtainable? (increasing bottem power without hurting top end much) Is my only option for this going to be changing out the prop or getting a different boat haha?

Personally, worry about the prop and worry about maintaining the current mechanicals/hull/etc.

Only thing really to think about: 4-bladed prop, SS prop. You have enough RPM that I don't think the top end (MPH) would take much of a hit.

 

[chevyracer5613]

Re: Newb looking for a crash course in performance for a scarab sprint

rbh- using the exhaust for the air pockets is a interesting concept, however I wonder how that could be obtained without dramatically increasing the back pressure.

Philster- Aside from going to a prop shop is there any way to mathematically determine which prop would be best suited for his boat? I am all for changing out the prop but i'd like to be able to make a spreadsheet of the compromises in order to determine which might be best all around. Also I understand that I stumbled in here asking to push a boat further then its peak and I appreciate everyones patience and ideas, and I am/was just hoping to find something that could help make this boat think its got a bigger motor and less weight haha.

Philster- I think I will start researching a 4 bladed prop, now if I might ask some theoretical questions about props what and why willa 3 blade act different then a 2,5,6 blade etc. Also aside from obvious size limiting factors is it best to have a prop with as long of blades as possible or is that a compromise also? Lastly, and I have not looked at props recently, but alot that I have seen have flat edges......do you think there would be any noticeable gain from putting a edge on the ends?

Another theory question, is it worth it at all to consider dual props (counter rotating or not)?

 

[rbh]

Re: Newb looking for a crash course in performance for a scarab sprint

Wide V, bottom has a lip so as you pass over the water the drag may pull the exhaust out easier.???
Sounds like this one is going to have to be desighned by a P-eng not me.

 

[chevyracer5613]

Re: Newb looking for a crash course in performance for a scarab sprint

rbh- I would like to think you could create a scavenging effect in the exhaust using that method as i'd imagine as the water flowed passed the opening it'd create a vacuum. However until you had sufficient speed to create the vacuum i'd think you'd see an increase in back pressure although you could always have a secondary exhaust that opens up if the back pressure gets to high. Lastly I wonder about water getting into that opening with the motor off.... I suppose it'd be find if you had a mechanism to block off the water flow when it saw a decrease in exhaust back pressure.

 

[haulnazz15]

Re: Newb looking for a crash course in performance for a scarab sprint

Variable pitch props would be an excellent solution, however I don't know of marine drives that use this idea except for tugs/ferrys and it's not quite the same application of the idea. Many aircraft use variable pitch props. You would like have quite an engineering issue on your hands to develop a variable pitch prop that would adapt to a Merc or Volvo outdrive. This would probably be a lucrative business oportunity if someone would ever design an outdrive that was capable of this. Would increase mpg's too since you could cruise at the same speed with lower rpms.

The difference in the 3-4-5 blade props is mainly in surface area. You are able to get better "bite" on the water with additional blades for good hole shot/acceleration, but at the sacrifice of top end speed. Many offshore racers use props like the Herring 6-blade props since they have enough HP to overcome and speed limitations.