Home › Forum › Ask A Member › Transom thickness for Big Twins
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olcah.
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May 25, 2015 at 8:02 pm #1602
Replacing the wood on the transom of my 56 Crestliner Commander, 12 ft. aluminum. I usually run a 1954 Big Twin 25 HP and plan to try a 56 Johnson 30 HP.
I plan to use white oak and wonder about thickness of the transom. The aluminum thickness is 0.085 sandwiched between the two pieces of oak. I would also like to put an aluminum plate 1/8 thick on the inside where the motor clamps screw down.
Looking at the motors I see at the top of the 25 HP motor clamp castings that the max transom thickness looks like 2" and 1 3/4 would fit nicely. The 30 HP could take up to 2 1/4" max. The boat has triangular aluminum corners that screw into the top of the inside transom wood piece. From where the holes are located in the aluminum corners it looks like the transom inside wood piece should be 1" thick. Then the sandwich would be 1" wood (inside the boat) + .085" aluminum + 3/4" wood (outside the boat) = 1.835 thick. Then add the 1/8 aluminum plate for the clamp screws = 1.960 ". That is the thickness of the old transom. It is somewhat difficult (#&#@*!!!) to put a Big Twin onto this thickness because the clamp ends tend to cock and hang up.
I really want to get this right the first time so would appreciate any advice.
1. Is 1.960 (really about 2") OK for a transom thickness for these Big Twins? What should I be using? Should I go thinner? I do run some much smaller outboards, 1947 and 1938 Sportwins 3 HP, but they look like 2" would be OK.
3. Is white oak OK for this? Boat spends all summer in fresh water.
4. Looks like the grain of the outside transom wood piece should be horizontal. Is that correct?
Thank you.
May 25, 2015 at 9:26 pm #16855I think a good quality Pressure treated 2 X 12 would work good
May 25, 2015 at 9:30 pm #16856A 12′ Crestliner? Hmmmmm maybe an 18 is the max I would put on that
May 25, 2015 at 10:11 pm #16858The catalog for 56 lists max HP for the Crestliner Commander as 30 HP. I used it a lot with the 25 HP last year and several members have 25 HP on those boats.
May 25, 2015 at 10:16 pm #16859quote 1946Zephyr:I think a good quality Pressure treated 2 X 12 would work goodNo pressure treated on aluminum. Also no galvanized. It will eat the aluminum. Look for exterior grade plywood or some oak.
May 25, 2015 at 10:25 pm #16860PT eats aluminum in the presence of an electrolyte, such as any water found outside a laboratory. If it’s dry, or separated electrically there shouldn’t be an issue. Up to you if you want to use it as your transom.
May 25, 2015 at 10:37 pm #16863Olcah,
I just did the transom on my 14′ MirroCraft using 2 layers of 3/4" quality plywood. I epoxied the two layers together, then coated all over with epoxy, then Spar urethane over that. I capped it with a piece of Stainless that comes down 4 inches on the inside and 1 inch on the outside. Total thickness is about 1 & 3/4". My 25hp RD Likes it! So do I.
I would think White Oak would be great at 1.75 to 2.00"
May 25, 2015 at 11:02 pm #16865White oak is fine as long as it is sealed on every side. Epoxy or CPES is excellent for this, but it has NO UV protection. If you use any epoxy product, coat with a UV stabilized varnish or polyurethane after the epoxy application. Check out James Town Distributers for a product called Total Boat Penetrating epoxy. Top of the line stuff I use for all my wood boatbuilding projects. It can be mixed and cut 50% with Acetone to give the ultimate in penetration in wood. Use this with Epifanes Boat polyurethane and you will never have to do it again in your lifetime. Of course if you want to do it on the cheap, and don’t mind doing it every 2 years or so just hit up your local hardware store and grab some exterior spar polyurethane. NEVER use pressure treated wood on aluminum. The copper sulfate used in the treatment process will eat holes in your aluminum in as little as 2 years.
May 25, 2015 at 11:57 pm #16877quote OLCAH:The catalog for 56 lists max HP for the Crestliner Commander as 30 HP. I used it a lot with the 25 HP last year and several members have 25 HP on those boats.That may be true, but that boat is 59 years old now. I don’t believe that it has the strength that it had when it was new. 😮
May 26, 2015 at 12:03 am #16878quote Outboard315:quote 1946Zephyr:I think a good quality Pressure treated 2 X 12 would work goodNo pressure treated on aluminum. Also no galvanized. It will eat the aluminum. Look for exterior grade plywood or some oak.
Good to know that. At least I’ll know, when I re-do mine. I just figured that since regular grade wood will rot out faster, the PT would last, but I stand corrected. White oak it is. 😉
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