1978 Seacraft 20MA newbie

[Bushwacker]

Originally Posted by Ironmanangler

Thanks. I plan closing the transom and adding a bracket. Want to power it with Yamaha 200 I4. I'm also thinking of going with a smaller tank then the original. I'm still in the planning phase. Thank you for the input. Keepem coming

FYI - Since you're in the planning phase, better plan on moving gas tank, batteries and console as far forward as possible! Don't know if you've read my post on the subject, but a bracket, heavy motor and smaller gas tank on a 20 are all moving in the direction to adversely affecting CG/balance, self bailing capability and ride of the boat! If CG isn't corrected, you can expect a min planing speed in the low 20's. As originally designed and powered, these boats would plane at about 12 mph, which is a big deal if you plan to run offshore in big seas!

[Ironmanangler]

Originally Posted by Bushwacker

FYI - Since you're in the planning phase, better plan on moving gas tank, batteries and console as far forward as possible! Don't know if you've read my post on the subject, but a bracket, heavy motor and smaller gas tank on a 20 are all moving in the direction to adversely affecting CG/balance, self bailing capability and ride of the boat! If CG isn't corrected, you can expect a min planing speed in the low 20's. As originally designed and powered, these boats would plane at about 12 mph, which is a big deal if you plan to run offshore in big seas!

How forward? I plan on going with a 50 Gallon tank, Original console and storing 3 batteries inside the console. I had planned on moving them forward. I was reading your thread just the other day but never finished it. I just don't want to go too forward and affect other performance aspects.

[Bushwacker]

What "other performance aspects" are you concerned about?

About the only performance advantages I can see in a bracket is the claimed 2-3 mph increase in WOT speed in flat water, which comes at the expense of ride and low speed planing capability. You gain WOT speed due to the CG shift caused by moving motor ~30" aft, which raises the bow, reducing wetted surface and drag. My experience is that when you apply the necessary band aids (AV plate fin & 4B prop) to regain low speed planing capability and good ride, you'll lose most of the WOT speed you gained with the bracket! So all the bracket really buys you is a bit more room in the boat AND the safety of a solid transom (which IS a significant improvement if you run offshore very much!)

What you're really asking about is how much should you shift the CG, if any? I would never claim to be smarter than Carl Moesly who designed these boats after decades of boat building, flying and racing experience, so I'd try to keep the CG as close to the design point as possible!

Here's what I'd recommend to calculate an approximate distance to move stuff:
1. Assume the as-designed CG was at the center of the fuel tank. (As a veteran pilot of military transports, Moesly knew how important it was to not change trim as you burn off fuel!)
2. Assume the design motor weight was 300 lbs, on transom, with a single battery at stbd chine just ahead of transom. The design "Motor moment" is (300 lbs) X (distance from transom to center of fuel tank).
3. Use weight of new motor (not the advertized "dry" weight, but "wet" weight including oil & gear lube @ 7.3 lbs/gal, plus weight of the cowling) plus the bracket setback dimension to calculate a new "Motor Moment".
4. The increase of the motor moment, in ft-lbs, is what you'll want to offset by moving batteries, fuel tank, console, etc. so by knowing what all that stuff weighs, you should be able to figure out how far you'll have to move it.

BTW, when looking at these newer "light weight/large displacement" 4-stroke motors, remember that they got most of the weight savings by just removing iron cylinder sleeves! That means that if, for whatever reason (clogged filter, stuck anti-siphon valve, air leak, fuel hose kink), you ever starve the engine for fuel that leans out & overheats a piston and scuffs a cylinder, all it'll take to fix it is a new block! The days of a simple bore & hone plus piston replacement are history for those motors. That's why the modern DI 2-strokes, which achieve even lighter weight, quiet smokeless operation, and low fuel burn without that compromise, are worth a hard look IMHO!

[Ironmanangler]

Originally Posted by Bushwacker

What "other performance aspects" are you concerned about?

About the only performance advantages I can see in a bracket is the claimed 2-3 mph increase in WOT speed in flat water, which comes at the expense of ride and low speed planing capability. You gain WOT speed due to the CG shift caused by moving motor ~30" aft, which raises the bow, reducing wetted surface and drag. My experience is that when you apply the necessary band aids (AV plate fin & 4B prop) to regain low speed planing capability and good ride, you'll lose most of the WOT speed you gained with the bracket! So all the bracket really buys you is a bit more room in the boat AND the safety of a solid transom (which IS a significant improvement if you run offshore very much!)

What you're really asking about is how much should you shift the CG, if any? I would never claim to be smarter than Carl Moesly who designed these boats after decades of boat building, flying and racing experience, so I'd try to keep the CG as close to the design point as possible!

Here's what I'd recommend to calculate an approximate distance to move stuff:
1. Assume the as-designed CG was at the center of the fuel tank. (As a veteran pilot of military transports, Moesly knew how important it was to not change trim as you burn off fuel!)
2. Assume the design motor weight was 300 lbs, on transom, with a single battery at stbd chine just ahead of transom. The design "Motor moment" is (300 lbs) X (distance from transom to center of fuel tank).
3. Use weight of new motor (not the advertized "dry" weight, but "wet" weight including oil & gear lube @ 7.3 lbs/gal, plus weight of the cowling) plus the bracket setback dimension to calculate a new "Motor Moment".
4. The increase of the motor moment, in ft-lbs, is what you'll want to offset by moving batteries, fuel tank, console, etc. so by knowing what all that stuff weighs, you should be able to figure out how far you'll have to move it.

BTW, when looking at these newer "light weight/large displacement" 4-stroke motors, remember that they got most of the weight savings by just removing iron cylinder sleeves! That means that if, for whatever reason (clogged filter, stuck anti-siphon valve, air leak, fuel hose kink), you ever starve the engine for fuel that leans out & overheats a piston and scuffs a cylinder, all it'll take to fix it is a new block! The days of a simple bore & hone plus piston replacement are history for those motors. That's why the modern DI 2-strokes, which achieve even lighter weight, quiet smokeless operation, and low fuel burn without that compromise, are worth a hard look IMHO!

I am concerned mostly that I would shift the weight too forward and the front of the boat would steer itself because it would dig.

I like the bracket not only for performance but I have two small children and would make it easier for snorkeling and sandbar days.

Thank you for taking the time to write all the above well explained information.

[Bushwacker]

Originally Posted by Ironmanangler

I am concerned mostly that I would shift the weight too forward and the front of the boat would steer itself because it would dig.

I like the bracket not only for performance but I have two small children and would make it easier for snorkeling and sandbar days.

Thank you for taking the time to write all the above well explained information.

If you had a later model 23 Sceptre made by SeaCraft Industries, I'd say you had a valid concern, as those boats appeared to be a bit bow heavy. Plywood was used for the core in the decks and bunks instead of the balsa core used on the Potter models. Restorations of some 23 Sceptre's on this site have also revealed pig iron pig iron ballast under the deck in the stern. However that's definitely not a problem on a 20, especially a CC model. My Seafari, with the cabin, bunks and head up forward, has more weight forward than the CC models, but I've never had a problem with bow steering or too much weight forward, even when I had a light 300 lb motor on the transom! In fact, on the half dozen trips I made to the Abacos, I've normally wanted even more weight forward, despite the fact that I already had all sorts of gear crammed in the cabin! On those trips I typically put a 70 qt cooler in the stepdown forward of the fuel tank to get more weight up front!

I do a lot of diving too and the swim platform on the bracket is great for that. The closer the platform is to the waterline, the easier it will be to use. Don Herman has the mounting height on his bracket dialed in very well . . . the swim platform about 2" above waterline, but that drops to zero when a couple of guys stand on platform.

[cdavisdb]

Reinforce Bushwacker on steering by the bow. I don't think these boats are likely to steer by the bow under almost any conditions. In my old Seafari 20(150 hp merc on the transom), we dived with 6 tanks in the well between the bunks, a huge amt of weight forward, and the boat liked it, tended to stay in the water going fairly fast into steep chop.. It would punch through chop that would hammer your kidneys otherwise. No bow steering tendency at all.

Just an observation: 200 hp seems like way more power than you need. My 150 merc was plenty for 3 divers, 10 tanks, 150 lb ice, 52 gallons of fuel, etc etc, 25 knots cruise in flat water. The boat was designed for more like 100.

[Franksanzo]

I wouldn't go any lower on the height of cap because or you'll get pryed off the boat in rough seas. my 21' has the cap with liner which has no room for "toe kick" yours has wider gunwale tops also nicer to walk on .close transom move console up to floor storage compartment with batt in front . Flip engine splash well so floor is extended to walk on keep live well under lean to or you could buy mine done and save the headache 😊good luck with the build

[Ironmanangler]

Originally Posted by Franksanzo

I wouldn't go any lower on the height of cap because or you'll get pryed off the boat in rough seas. my 21' has the cap with liner which has no room for "toe kick" yours has wider gunwale tops also nicer to walk on .close transom move console up to floor storage compartment with batt in front . Flip engine splash well so floor is extended to walk on keep live well under lean to or you could buy mine done and save the headache 😊good luck with the build

I can't trim the cap more then it already is and wouldn't even if I could. I'm not happy the previous owner cut them but I'll live with it.

Yours is very nice but I'm going to stick with the experience of a rebuild.

[Ironmanangler]

Anyone know why the 20MA has a drain whole through the hull for the front compartment? My first guess is that someone had used it as a bait well. I would assume the storage compartment would drain into the hull but this one has the hole right through the hull

[Vezo, Part II]

My 1979 MA has the thru hull fitting/drain. Quite certain it is factory and with a screen fitting would work well as a baitwell. Hope this helps.

Vezo, Part II