How Fast Should I Be Going???
 

Posted in the Scarab form but nobody got back with me :( so I thought I'd post here.

21' 1987 Wellcraft Scarab I, 350/260HP Merc Crusier with Alpha Stern.

At WOT I'm running almost 5K RPM's. Top speed trimed out I have reached was 44 MPH via speedo; I don't have a GPS reading. That was with a load of 3 adults and two small kids. I also had a large "whale tail" fin on the back that I'm sure wasn't helping much.

The current prop that is on it is a Quicksilver 48 3 Blade. It also had the numbers 16316 45 19P on it. In relatively good shape. I also believe it is stainless steel. RH motion.

How fast is one of these boats supposed to go?. I've seen cruising speeds of 40MPH to 50MPH with WOT speeds of 52-63MPH. I just want to know how far off off top end I am vs. stock so I can start to make my performance mods. First thing will prob be a new prop; then a 4 barrel carb up top. Then???? :evilb:

Please, any help would be great.

Re: How Fast Should I Be Going???
 

I assume you are talking about the 21ft? I had one a couple years ago. It was a 1984 with the 350/260 Alpha. I believe i was turning a 19p 3 blade and running 54-56 @46-4800rpm on a handheld gps. Its not going to be a real fast boat, just enjoy it. They sure ride good for a 21'.

Re: How Fast Should I Be Going???
 

I think you will find that doing "here and there" performance mods really won't get you much, if any speed. I had a 20ft Formula Spider years ago that had a tricked carb, prop, and breather on it. The boat ran basically the same as when it was stock. Unlike cars, the weight of the boat will command at least 50hp more to see any mph increase no matter how light it is.

If you want to have a little rocket I would look into a procharger, or whipple setup. Those will give you immediate results that you will love and you won't break the bank or the boat doing it.

I would contact "Clay Washington" here on the board for some details. He has a single engine Crown Line with a whipple and seems to love it.

Re: How Fast Should I Be Going???
 

My friend has a '87 210 Elite. With a 350/260 he was going around 50 MPH. He put in a 373 with 320 HP and he is going 54 MPH.

Re: How Fast Should I Be Going???
 

So do you all think 55-60 is easily possible with prop/carb/intake?

Re: How Fast Should I Be Going???
 

55 Maybe, 60 doubtful. My friend was expecting better numbers. The Elite and the scarab probably have the same bottom. I would enjoy it the way it is and start saving or searching for a "Fast" hull.

Re: How Fast Should I Be Going???
 

I'm a little confused by your post...but if you are only seeing 44 mph on your speedo, I would be willing to bet that your speedo pickup tube is bent or partially clogged. get a handheld gps and check it again!! :cool:

Re: How Fast Should I Be Going???
 

So I will aim a stock speed of 50; and a modded speed of around 55 for my goals. Does this sound about right?

Will also go and borrow a GPS on the next run to see what numbers I can come up with.

Plan on being on the boat most of the weekend if it is nice.

Re: How Fast Should I Be Going???
 

Agreed :cool: I had an '86 and loved it to death. You could regularly catch me flying at LOTO on Holiday weekends whenever the "OBNOXUS" sled was broke or at Lake Perry, KS :evilb:

Re: How Fast Should I Be Going???
 

I kinda doubt it with just a carb and intake. The prop might give you the most results, but probably not 5mph. 5mph is quite a bit of increase in a boat.

Re: How Fast Should I Be Going???
 

Ferraristyle,
There is a 85 scarab 21 on e-bay right now that has the same motor guy has it listed as going 50mph so your speedo might be off a bit. I wouldn't expect much more with that set up.
Good luck and good boating. :cool:

Re: How Fast Should I Be Going???
 

I use to have an early 19 foot powerquest back in the early eighties with a stock 260 with thru-hull.Merury came down and propped it and it radared right at 59mph and when i raced all the 21 scarabs i had about a half a mph on them . I used to race a lot of them and they were all consistent in speed,so i think if your 260 is still healthy you should be right at 58.

Re: How Fast Should I Be Going???
 

That whale tail is probably costing you some mph. Good for getting up on plane if thats a problem but it's a top end killer.

Re: How Fast Should I Be Going???
 

I have some guys telling me 50MPH and others saying 57/58 MPH. That's a HUGE difference. :eek: What is the correct number? If the number is 50 then I'll probably be good on this run with the whale tail off and a lighter load (1 less adult; 2 less kids = 275lbs). I'm thinking that will get me close to the 50. But if 57/58 is correct then I have something wrong somewhere.

Is there any factory literature or numbers to verify what STOCK top speed was/is? Maybe some of the 21/22' Scarab guys could chime in and get a more accurate answer.

Anybody with the exact WOT top speed on this boat would be helpful.

Re: How Fast Should I Be Going???
 

Trust me on the 58 with two people and half a tank,but you should be turning 4600 not 5000 .Not enough prop?

Re: How Fast Should I Be Going???
 

Which leads me into the question what size and pitch prop. I posted a link over at Prop Talk, but no one gets back with me. I don't know the gearing on the boat, but I have a Stern Drive "Chilton's" which I'm sure has it in it, just don't know what info I'm looking for.

Sent a PM to Julie over at Thottle Up but she hasn't gotten back with me either.

Any other prop vendors or prop people I can talk to or PM. Belive me, if the prop is what is holding me down; I'll be buying a new one tomorrow, just got to find out what size and pitch.

Help, direction, and comments appreciated.

Re: How Fast Should I Be Going???
 

One other thing to keep in mind also because I ran into this also is your tach might also be off. I had a 1990 23ft wellcraft nova that I could just never get dialed in right turns out just the tack was bad wouldn't ground right. just my .02 sometimes it is the little things.

Re: How Fast Should I Be Going???
 

I agree on the little things,could be you are running 55-58 and your speedo is lying which most likely it is .I have never had one half way accurate on a small v such as yours because most of the time your not in the water but on the drive at wide open.Do a gps reading first.

Your drive should be a 1.5 to 1 and i think the prop i ran was a 19x19 cleaver but not positive i'll check tomorrow.

also if the motor has some hours on it can run smooth but not have enough compression to produce the full 260hp.

Re: How Fast Should I Be Going???
 

tommy

Thanks for the help so far. Just sort of disheartening that a relatively small boat with such a big engine only turns out in the 50's. Guess the hull and weight have a lot to do with it. Man, if this were a car I'd have no problem getting speed out of it, but I know cars, not boats which is an entirely different game.

Before last weekend I did a complete tuneup on the engine, everything except for timing and alignment which I'm getting done in two weeks. But I do have everything done including synthetic oil, new battery, plugs, distributor, wires, belts, fluids, etc.

I'm splashing the boat this weekend; trying to find someone with a portable GPS that will let me get a reading (anyone that lives near Annapolis, MD and wants to help me out with this PM me). Does anyone know of a place that rents one for a day?

I know for a fact that the large whale tail was holding me back some; but I didn't want to take it off before I knew what it did. (It's off now) Also know that I have to recheck my speedo tube, as last time I was out it kept folded up on me twice. Maybe this is the problem?? Man that would be an easy fix.

I also know that I was just about at 5K RPM wise at WOT; which is where my concerns for the prop come in. I should be 200-400 less. Bare in mind, since my boat was a movie/show/stunt boat, (I have the Miami Vice Action Spectacular Boat previously owned by George Barris of Batmobile fame) the boat has probably been set up for low speed not top end.

If worst comes to worst I'll have my friend run beside me on my 1100 Waveraider. On smooth water I easily hit 64 MPH with that so I could probably get a good estimate.

Finally, I'm bound and determined to change the prop. I know that this is part one of my issue. However before I go and spend $500 -$700 on an new prop I want to know what to get.

Re: How Fast Should I Be Going???
 

pick up a cheap handheld gps, that will be your best tuning agent.... probably can find one for 100 bucks or so....

then test, test, test.... borrow some props if you can first before you buy anything... then change one thing and test some more....

nobody is gonna be able to tell you an exact speed because of water conditions, boat variance, driver inputs, weather conditions...

your next step is make sure you are making 260HP.... tune up, compression check...

Re: How Fast Should I Be Going???
 

I have a 88 Chris craft limited with a 260 in it,alot bigger boat and with a 21p Mirage and a holley 650 and aluminum intake,it goes 48.9 on gps and that is the only way to get a true mph.I have never seen a boat speedo read accurate.I used to have a 20' sea ray with a 260 and it went 51 on gps with a 21p prop and stock engine,I would say you should be around that as the boats should weigh around the same,also with a similar hull desighn,but water conditions can reflect on that.