1986 Scarab 1 Bow Lift Issue

This is the running surface of a 1986 Wellcraft Scarab 1. The same hull design was used for years across several Wellcraft models. The design uses two full length strakes terminating at the transom in addtion to a partial pad that is slightly rounded at the keel. The hull is unresponsive to trim and rides flat and wet at WOT. This seems to be a systemic issue with this hull configuration. Removing 350 lbs of weight from the engine compartment only aggravates the issue. I've owned a number of high performance boats including both pad and round keel deep V's. but this is the only one that has the inner strake extend to the transom. The pad does exhibit some hook, which is not condusive to bow lift however, others have removed the hook with only a modest bow lift improvement. IMO, the inner strake is lifting the stern at the detriment of forcing the bow down. I don't require as much stern lift having removed so much weight from the engine compartment and I see an opportunity to reduce that amount by another 80 lbs.

I'm thinking of shortening the inner strake to something similar to that of my Nordic Heat, that has no issue with insufficient bow lift. These Wellcraft's become somewhat ill handling over 70 mph with both chine walking and bow steer. I'm speculating that this extended inner strake was designed to keep the stock boat on top with stock power at lower speeds, but is counterproductive when seeking significant speed increases when adding HP.

Opinions or experience?? It's a pretty gutsy move to start grinding off sections of strake on a hypothesis.

 

What else are you going to do?

On this Forum, you're lucky to have a hull to modify, most are in the design phase.

What are the other owners doing to alleviate the problem(s)?

Oh, and welcome to the Forum.

 

Welcome to the forum Ed.

This is somewhat strange on a very high speed monohull:


The objective of a planning hull is to promote the separation of water flow, to generate the dynamic lift.

The water flow in this region would be something like this:


This flat strake, will create a lot of flow separation but not the good kind - turbulent flow, between its edge and the chine close to it.
As such it will limit the effects of this chine:


It will be in aerated flow, a mix of water and air.
So it is highly like this region:


Is not contributing effectively.
It would suggest that this is a more appropriate modification:



Making the keel line a true Vee, to enable the separation of flow and to promote the flow of water aft and transversely.

My 2/c worth.

 

I have not seen anyone have any real measure of success with this bottom design other than overpowering the boat with horsepower. Even then, the handling issues are so significant that owners are forced to use oversized (for the hull length) trim tabs for some measure of stability.

 

I’m thinking that the original intent of the design did not consider 70+ mph speeds!
Looks more like it was designed to hold on plane with low speeds.
That aside, straightening the hook in the pad would be a logical first step toward allowing the bow to rise at speed.
Also some propeller tuning might help, many propellers are crafted to help lift the stern.

 

I've always thought along the same lines. I've removed hooks from other hulls and have a selection of props with a fair amount of rake, but I'm fighting a pleasure boat bottom design. I'm just considering changes that might free the hull up with the additional power I now have. In comparison, my old 21 Superboat had a V shaped pad that ran over 50% of the keel length. You could fly the bow as much as the trim button and your courage allowed. I'm still lamenting selling it!

 

Ad Hoc, I would agree with you based on my experience with another pad V bottom I owned, a 21' Superboat that was first introduced as the George Linder designed Challenger 21. That boat had a pad that was V shaped vs. flat. It provided so much lift that the inner and outer strakes were virtually out of the water at high speeds. The boat ran close to 90 MPH with a 300X Pro Max outboard. It was a balancing act to keep it on the pad at those speeds, but with driver input, you could do it. That pad exceeded 50% of the keel length. This boat does not have a traditional pad, as I have come to know them on higher performance V bottom boats. It's more of a flattening of the keel.

 

Looks like a great boat and project!

scarab bottom design is pretty well proven high speed configuration,

what power are you using, what speed are you achieving? What’s your drive height (x-dimension)?

 

I'm running an all aluminum 450HP LS3 with an Alpha One, Gen 1 (1.5:1) spinning a 3 blade Mirage. I'm on the ragged edge of breaking this drive. I really baby it. I also have a spare Bravo 1 (1.65:1)and a Blackhawk (1.65:1). The propshaft centerline is 5.5" below the pad, essentially the stock Wellcraft set up. Full hydraulic steering. With a light fuel load, it will run 70 GPS with just me onboard. Fast to some, slow to me. I could raise the X on the Alpha One or install an Alpha SS, but the drive internals and stock propshaft are already limiting factors. I'd love to raise X 2" and install the Blackhawk (1.65:1) or B1, but the hull has little bow lift and is not responsive to trim with the current setup. I'm used to flying pad V outboards and this I/O just rides flat. Over the winter, my plan is to remove the hook in the pad and do some additional prop testing next spring, but I'm not optimistic the results will be significant. One of my old boat builder customers is about two hours away, so weighing the boat is also in the plan. As I initially posted, I've taken a lot of weight out of the engine compartment with an opportunity for another 60-80 Lbs.

I was actually going to reach out to you a while back , Jim, as I have read some of your published material online.

 

Your comment "One of my old boat builder customers is about two hours away, so weighing the boat is also in the plan."

Columbiana has a public weight scale that will save a 2 hour drive.

Weigh the entire rig, boat + trailer and tow vehicle
remove the boat, weight the boat trailer and tow vehicle
The difference is the weight of the boat

 

I use the certified scale in Canfield when weighing trailers (Ohio really puts boaters through the ringer if you lose your registration). Subtracting the trailer weight will indeed give me the weight of the boat which will give me an indication, if nothing else, if the boat is hiding water. A boat builder typically has a means of suspending the boat with scales at both ends. I'm particularely interested in finding the CG of the hull. If I had an adequate gantry system in my shop, I could do it with a fulcrum.

 

Hi Ed - i don't recall dimensions of the 21Scarab bottom, but the flat pad is a performance advantage when set up well. You're right that this boat will be quite sensitive to weight, so even an estimate of current weight would lead us to a prediction of possible top speed available. if there is hook in the pad, that could add to trim issue and performance too. i think that your x-dim of 5.5" below the pad is quite low, will affect top speed and bow-down trim moment. Raising drive unit will help. Don't know what size prop you're using now, but if your Alpha has 1.5:1 gears, then you should be able to see high 70's with 26" prop. maybe try Max5 prop for more bow lift?

call any time if you'd like to talk.

 

Speaking of boats from this era and making them go fast, I owned a 1986 Chris Craft Stinger 222 for a number of years which I loved and came across this video recently on YouTube and was astonished that these guys have hit 123MPH in the slightly smaller Stinger 202 model.

And surprisingly, it actually looks pretty stable at speed!

 

Jim, the X is way too low IMO. I ran my propshaft even with the pad on my old Superboat

I always loved boats that don't look the part when laying down big numbers. There's also a guy in Norway running an LS motor in a Fjordling over 90. Both the Stinger and Fyordling look as if they are on rails at speed.

My go to setup will ultimately be the Mercruiser Blackhawk surface drive with a dual turbocharged LS3 in front of it. I just need to get the hull as good as it can be. That begins over the winter.