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.