Sorensen long haul center fin

[Jippe]

Plans call 9"×3" center fin but theres nothing about that where it should be, I need advice for that.
If I dont find just that size fin, is it better to use little smaller or bigger?

[champ20B]

Plans call 9"×3" center fin but theres nothing about that where it should be, I need advice for that.
If I dont find just that size fin, is it better to use little smaller or bigger?

Jippe, you can buy extruded "T" aluminum by the foot online or at a nearby metal supplier. You can make your own fin this way easily.
Afterward, you need to polish it and sharpen it from the leading edge along the bottom edge to just where the trailing edge starts. Make sure it is perfectly aligned with the keel (perfectly, as not even 1/32" off).
Now......heres how to place your fin to start for testing. Being that your boat is a center fin traditional roll-up, the way is to locate the point of balance. You must put the motor on the boat, rig it up with all hardware, fill up the gas tank 3/4 full, and get in it with a pipe or mop handle across under the hull like a sea-saw. (It would be good to have it up on a platform or table so the motor can hang down.) What you want to do is to locate the pivot point (pipe or mop handle) forward or aft until the bow and transom are balanced level like having equal weights on a sea-saw. Again, this is with your weight in the boat fully rigged. That would be the approximate ideal spot for your fin. It is a good idea to be where you can move your body forward of the fin location in your boat in case you need to when turning to plant the nose a bit. But that depends on how it handles with your body weight.

You might find that the fin would be about 1/5 to 1/4 the boats length from the trailing end of fin to the trailing end of transom bottom. However it could be a little more still. Many guys on here don't like to share much of this kind of info or just don't really know, as I found out when I asked about fins once. I hope this info will help.
Just be careful not to damage the hull. You might want to make a narrow flat padded board (2 X 4) with a mop handle nailed down along lengthways of its center. Put the hull on the flat board with handle underneath.

[Jippe]

This is good info, thanks. Is it center or maybe offset left?
I have boat almost ready and it is still in my garage on the table.
Painting, varnishing and rigging is not done yet.

[champ20B]

This is good info, thanks. Is it center or maybe offset left?
I have boat almost ready and it is still in my garage on the table.
Painting, varnishing and rigging is not done yet.

Jippe, if you want to get right and left turning capability, it will go center.....however, in the past they would mount the fin left a bit. It would be between the next stringer and center keel I think. Glen-L boat gear also sells fins. They have a high speed shark dorsal style fin that might work well on your C-class runabout. It is a cast unit that may require milling on its mount plate to make sure its 90 degrees. It would need to be polished and sharpened to your liking as well. You can make a regulation type (long rectangular w/rounded corners) using 1/4" thick "T" extruded aluminum as well. Don't worry though about drag. Fins really don't have enough drag to amount to anything as long as it is in line and true. A fin that stays in contact with water though, at speed, is a good fin in my opinion.

[champ20B]

Jippe, I forgot to tell you also, that mounting left is mostly for left turn "closed course racing". It reduces right turn stability....

[Krazy Karl]

On my Long Haul which I didn't build has 2.5" x 7" rounded fin positioned 29.5" from the rear of the planing surface. It is 9.75" from the edge of the left side planing edge and 20" from the right side. This boat is the second Long Haul generation with the 2 angle non-trip chines, a vast improvement over the first version. It is a lot better handling boat than the first which I started with. I would NOT center the fin on one of these boats as they will turn hard to the right when they hook. The counter-rotating prop will add to right hand turns significantly enough to throw you out if you are not expecting it. A
"D" Merc on one these will throw you out on a regular basis if you are not careful.
These are fabulous Marathon boats and handle rough water like a dream. I have run Top 'O twice with my 1st generation with a Yamato and want to go again with my second. By the way Darrell is a good friend who designed these boats based on a Hill boat if I remember correctly.
kk

[champ20B]

DAM, IM GOOD!!! 29.5" forward? That is just what I estimated!!! I said it would be about 1/5 or so the length of the boat forward from the transom!!

But I would place it balanced with the motor and fuel tank location etc your running Jippe. Just find the balance like I recommended to get it right for your particular set up and weight, to get it just right, as far as linear placement goes. Adjustments can be made afterward from testing if necessary (forward/back-center/left).

I really didn't know that such a great (left/right turning) marathon boat would need a left fin. That's a real surprise!!! I saw pictures of these guys in freeze frame after jumping a wave with similar boats, and I swear I saw those fins right in the center of the exposed planning surface. I don't see roll up "C" class though in closed coarse racing, so I never seen a modern "C" boat with a left mounted bottom fin. They were around a long time ago and I seen old pictures of them. All I ever see are side fins now days.

[Jippe]

Point of balance is 29,8 inch from rear

Next question is yamato 302 trim angle. With stock adjustment system I cant get it even paraller.. It is negative with max. setting.
What is good angle to start with? Paraller to bottom?

[zul8tr]

Start parallel and pick a height like 1" below bottom for rear center of the prop shaft. Get speed and rpm and note the attitude of the boat: is the bow up or low to the water? If low kick it out a bit, if high kick it in a bit. Note speed and rpm and ride characteristics each setting. Then try higher with level shaft and repeat above. Note water conditions will change things for all settings as well as different props, many combinations so have safe fun.

You also need to decide what your goal is for the boat: top speed, good turning, etc, a compromise will be needed to achieve several outcomes.

[Jippe]

First test drive done. Shaft is paraller and 35mm below.
Boat porpoising very much (bow up and down). When moving my weight to forward it helps little bit but not much.
What causes this? Prop, trim angle, motor height?

Water was dead flat and prop 3 blade dewald cleaver.