21 ft Cat advise

Hello gentlemen, this is my first post here but I lurk occasionally. I recently have acquired a new to me boat and in the pursuit of never leaving anything alone, I’ve been thinking of ways to make it better. It’s a 21 ft cat that is a copy of a 21 Skater. These boats were designed in a time of 2 stroke outboards that weighed 200lbs or more less than the L6 Verado it’s currently got hanging off of it. Some of them however, less than 10 total, got twins. So even with lite motors, we are talking about 800 lbs minimum.

So for my question, I’ve been considering the idea of removing the transom notches on the boat but want some opinions on how that would affect the handling and overall attitude of the boat. My understanding of the notches was to help get water to motors mounted behind the sponsons, but in this case, no motors there. They also shorten the running surface a bit which I feel would be beneficial to have more at that point to help lift the hull up. They would also add a bit of flotation to the boat.

The idea would be to fill them up to the transom and just match the running surface all the way to the transom.

Thoughts? Ideas?

Leave them.

Or at least drive around and gather some performance data.
Then, if you just can't resist, fill them in.
Let us know what happens.

BlueBell said:

Leave them.

Or at least drive around and gather some performance data.
Then, if you just can't resist, fill them in.
Let us know what happens.

Click to expand...

I have had the boat out several times and a 21 skater is well documented as to what it can do with different power and setups. This same boat with a 400r went 108 with the previous owner.

I’m more interested in understanding why they added the notches in the first place. What benefit would they offer for a single engine application?

You could install trim tabs in the notches.

Flight Risk said:

....I’m more interested in understanding why they added the notches in the first place. ....

Click to expand...

They look suspiciously like a recess for trim tabs. Given their "exact" nature and location.

Or do as Bluebell suggests.

We tested a 16' free-running scale model of a 72' high speed (30 knots design speed) catamaran yacht, with and without transom trim tabs. Similar assymetric planing hulls as the ones in OP. The trim tabs provided a big improvement in the time and power required to get the craft on plane and reduced the minimum planing speed significantly. As the yacht's displacement grew over time, as so often happens, it reached the point where it could not even get on plane without a lot of trim tab lift.

You can't see it in this picture, but the outboard engine on the scale model is mounted on a thrust measurement plate so we had accurate measure of the drag for various speeds and trim tab settings, in addition measuring speed and trim.

I included a picture of a tab installed on the yacht. The trim tabs were actively controlled to dampen pitch and roll motions as an added benefit to helping plane out the boat.

I work at Skater and the 21's were way before my time but we still build similar transom notches into all of our outboard boats.
My understanding of their purpose is that they help keep the bow raised at speed in rough water. A full-transom outboaed boat has a tendency to trip on a wave, forcing the bow down and stuffing the boat into the next wave. This is of course comparable to driving a car into a concrete wall with no seat belts and should be avoided. The heavier weight of inboard engines limits this risk relative the much lighter outboards, so most I/Os do not get the notches.
I don't recommend filling in these notches, the gain in buoyancy only has an advantage with the boat at rest and eliminating the notches would increase the opportunity to stuff the bow as described.

Thank you for the response. That makes a ton of sense.

Maybe you could help answer another question for me. The 21 also has that small diverter in the center of the tunnel. Would you happen to know the reasoning there and if it was ever tested without it? I’ve heard it was to help the boat plane off but maybe you can get the answer from the man himself. Thank you

Balsabutcher said:

I work at Skater and the 21's were way before my time but we still build similar transom notches into all of our outboard boats.
My understanding of their purpose is that they help keep the bow raised at speed in rough water. A full-transom boat has a tendency to trip on a wave, forcing the bow down and stuffing the boat into the next wave. This is of course comparable to driving a car into a concrete wall with no seat belts and should be avoided.
I don't recommend filling in these notches, the gain in buoyancy only has an advantage with the boat at rest and eliminating the notches would increase the opportunity to stuff the bow as described.

Click to expand...

So that diverter is colloquially known as a "nut-sack" around the shop and its use was early and relatively brief, but again, the single outboards were way before my time so my knowledge of it is pretty much hearsay. I've only seen a couple 21's come in for resto/repower work.
Certainly said diverter could provide some lift at low speeds but is well above water with the boat at speed.
I think it was more intended to break up waves and disrupt water ahead of the outboard to reduce impact stress on the single motor hanging in the center of the tunnel otherwise unprotected.
The diverter was removed at some point in the life-cycle and was only used on the singles, so 18s and 21s.
If I think of it Tuesday I will ask Pete and get a more definite answer.

Flight Risk said:

Thank you for the response. That makes a ton of sense.

Maybe you could help answer another question for me. The 21 also has that small diverter in the center of the tunnel. Would you happen to know the reasoning there and if it was ever tested without it? I’ve heard it was to help the boat plane off but maybe you can get the answer from the man himself. Thank you

Click to expand...

The fact that you get to play with those toys is bad ***! Getting an answer from Pete would be awesome. I mainly ask because with this heavier 4 stroke, unless I’m really moving, that diverter must be in the water acting like a tab lifting the stern but also dropping the nose. I wonder what it would run like without it. I see boats like the Argo 23 that’s a Single outboard but straight tunnel and they are fast!

Balsabutcher said:

So that diverter is colloquially known as a "nut-sack" around the shop and its use was early and relatively brief, but again, the single outboards were way before my time so my knowledge of it is pretty much hearsay. I've only seen a couple 21's come in for resto/repower work.
Certainly said diverter could provide some lift at low speeds but is well above water with the boat at speed.
I think it was more intended to break up waves and disrupt water ahead of the outboard to reduce impact stress on the single motor hanging in the center of the tunnel otherwise unprotected.
The diverter was removed at some point in the life-cycle and was only used on the singles, so 18s and 21s.
If I think of it Tuesday I will ask Pete and get a more definite answer.

Click to expand...