18' SF Performance Questions

[otterhound]

I finally launched a 1978 18' SF I picked up last fall. It has a 1992 Evinrude 120 hp which after rebuilding the carbs seems to run well. The compression checked out at 128 psi per hole so it seems the bones are good. The prop is an aluminum 3 blade in good shape and it's marked 13 1/4 x 17. The max rpm I've seen is 5000 at 37 mph and 4000 will give me right arround 26 - 27 mph. It also seems to fall off plane around 18 - 19 mph. The 5000 rpm max seems low to me although I'm happy with the cruise numbers. Is it possible that the motor is over propped? I'm willing to grab another prop but unsure of which way to go. Also the boat does not have trim tabs. Thanks!
Rod

[Snookerd]

Sometimes the coils are ok at idle but are failing to produce the solid higher rpm spark....did you look at the plugs to see that they look good? Prop sounds ok. I had 2 v4 130 yamahas on 2 different 18's. 1 prop was a 17 and one was a 19. The 17 was the best one I used.

[Bushwacker]

I used to run a 13 3/4 x 15P prop on my 20' Seafari when I had the 115 on it, and it would turn ~5500 pretty much regardless of load. Your 18 is quite a bit lighter, so I would think it could spin a 17" prop up to 5500. I think 5000 rpm (if it's an accurate number) is probably too low for your motor. Check your throttle linkage and cable rigging to make sure the carb butterflies are full open at WOT. You're lugging the motor if it won't reach max rated rpm, and that's harder on the motor than letting it over-rev! The crank on those V-4's is only about 6" long and very stiff, so it's good for about 9000 rpm! Rev's won't hurt it but lugging the motor increases combustion pressures and temperatures and loads on the pistons and bearings and the chances of detonation/pre-ignition, so better to let 'em rev! Don't know what the rated rpm on your motor is, but I'd think you'd like to see at least 5500 with whatever load you typically carry. But before you go screwing around with props, I'd do a couple things:

1. Analog tachs are notoriously inaccurate, so it may be reading low. I'd try to have a dealer check it. Some dealers have a dyno they can hook up to the motor and run it up to it's max rated rpm under load and check the ignition system and tach reading while they're doing it. I had that done to my old motor after it had sat for about 5 years, just to make sure it was in good shape before taking it off shore.
2. Most dealers mount the motors too low! Your AV (Anti-Ventilation) plate wants to be above solid water when you're on plane and fully trimmed out. If you can't see the top of your AV plate, the motor needs to be raised, which will cut drag and maybe buy you some more rpm. On most 20' SeaCrafts, this will put the AV plate about 1-1.5" above bottom of boat. A good SS prop with some cup in it can also run higher on the transom without blowing out in turns.
3. Your min planing speed is pretty high. Most 20' SeaCrafts would plane at about 12 mph if powered with the 300 lb motors they were designed for! That's a big deal because the 18/20 is pretty light and will start to go airborne in 3' seas at about 20 kts, so if you run offshore very much, it's nice to be able to hang on plane at lower speeds. Some props also develop more stern lift than others depending on blade design, and that will help reduce your min planing speeds. Bass boat props are designed to lift the bow for high speeds, while props for offshore fishing/cruising boats are designed to generate stern lift. 4 blade props are particularly good at providing stern lift.

[otterhound]

The tach could very well be off, I will try to find another to swap it out with. I have an account with a local marine supplier so I can get pretty much anything. I did raise the motor so the AV plate is 1 1/2" above the keel, it was very low when I first got it. Today with daughter running the boat I looked and the AV plate is right where it should be on plane and trimmed. Thanks!
Rod

[otterhound]

Just took a quick look and I have 3 other tachs to choose from so should be able to eliminate that variable. It pays to have spare boats kicking around
Rod

[76Red18]

Sounds like it's the prop design if you fall off plane at that high speed. I use a 14 1/4 x 15, 4 blade bay pro 3 on my 18 with a 375* 150 Johnson. Fully loaded, I see 5500 rpm and 40 - 42 mph. I can plane around 12 mph. This prop is a stern lifting design.

I had a 14 1/2 x 17 viper that would achieve the same top end but would not plane below 20 mph. This prop is a bow lifting design.

Most stock aluminum props don't provide much stern lift. I would try a 13 x 15 stern lifting prop. Stock omc sst, mercury black max or yamaha cleaver props have that design.

[otterhound]

Thank you for your suggestions, after I check a few of the simple things first I think give a four blade a try. I can get a Michigan Vortex 4 blade 13 3/8 x 15 or 13 x 17.
Rod

[Bushwacker]

The Vortex is an aluminum prop, so it can be easily repitched to fine tune it. I bought a 4B 14 x 20P Vortex and had it repitched to 14 x 18P to get the rpm up. It dropped my min planing speed with the 150 from 22-23 mph to 12 mph! If you find a Vortex that works, it's easy to upgrade to the equivalent SS Apollo 4B prop, which is a very efficient prop, and typically about 1/8" larger in diameter for the same pitch, since the thinner blades are easier to turn. Both props use the Michigan 2-piece SS/plastic hub system, which can also be used on PowerTech props. It looks similar to the new BRP hub system and is much more robust than the Merc 2-piece hubs, which are known for failures of the plastic hubs.

I'm guessing those extra tachs you mentioned are all analog tachs, which can read either high or low. You can change tachs and get a different reading, but you still won't know if it's accurate unless a dealer checks it! I installed a new 115 on my boat when I bought it in 1975, along with a new tach. When the dealer checked out the motor, he told me the tach was off by several hundred rpm, but he agreed to swap it out since I had bought it from him.

[otterhound]

Good info on the props. I can get the 4 blade Vortex very reasonably so it would be good to have a spare anyway. Not knowing anything about propellors I'll give the 13 3/8 x 15 a try. Also I'll give the local dealer a shout and see if he would put my antique through a stress test. Problem is everybody out here is spoiled and nobody wants to look at the old stuff anymore lol! Thanks!
Rod

[Bushwacker]

Hey Rod,

I think I spent around $120 or so for the Vortex prop, not counting the hub assembly, and that was at West Marine which is not known for low prices! If it over revs, a good shop can add more cup to it. PowerTech and Solas also make some good SS props, but you'll probably spend over $350 for one of them!

The "stress test" is a good idea because not only will it be a good check of the motor, knowing what the tach error is could save you from buying the wrong prop! Regarding old boats, yea, the newbies don't know enough to appreciate the quality or sophistication of the SeaCraft hull design, but I think you'll find that SeaCrafts are highly regarded by most old timers who really know their stuff! One of the early brochures stated that very few SeaCraft buyers bought one as their first boat, as they were relatively expensive when new compared to the competition. It also said that "The price of a new boat tells you what the builder thinks he can get for it; the price of a used boat tells you what other folks think it's worth", adding that the Seafari 20 had the highest resale value of any boat in it's class at the time! (mid '70's) Denny