Engine height

Starling · #1 03-09-2022, 04:24 PM

Hi,
I've currently done a partial restore on my 89' SF20 and it seems like I'm pushing a lot of RPMs for cruising on plane. I'm fairly new to boating and have used a newer boat with a flat back that got on plane and stayed there at much lower RPMs than this one. I know that the Seacraft's have no flat back but the motor seems to be using more RPMs than I expect. Right now I have about 50 gallons of fuel, a T-Top, two batteries under the helm, 3 batteries in the fish box, and it's about 4800 cruise at 20-22mph. Is that normal? My prop is a suzuki 3rx 14x16p. - Thanks

bmajvi · #2 03-10-2022, 08:21 AM

Well, 20 years ago I put a new DF140 on an 18 SF, and got 43.8 mph by GPS at WOT with a Suzuki S/S 14x20 prop, so it sounds like your prop may be way off?

Starling · #3 03-10-2022, 01:13 PM

Yea, the more I read the more it looks like I'm at minimum 2 off on the pitch. I'm considering taking the prop to a shop and having them adjust it to 19. 14x20 seems incredibly common for that hull and motor but maybe I'll start there.

uncleboo · #4 03-10-2022, 02:40 PM

Quote:

Originally Posted by bmajvi

Well, 20 years ago I put a new DF140 on an 18 SF, and got 43.8 mph by GPS at WOT with a Suzuki S/S 14x20 prop, so it sounds like your prop may be way off?

I have a 13-3/8" x 21P prop on mine and get about 40 mph wot. Anti cav plat about 2-1/4" above keel.

Fr. Frank · #5 03-12-2022, 10:59 PM

Quote:

Originally Posted by uncleboo

Anti cav plat about 2-1/4" above keel.

That's about right, but JFTR, that flat surface on the lower unit above the prop? It's an anti-ventilation plate, not an anti-cavitation plate.

Here's a greatly simplified definition of each:

A propeller CAVITATES at high rpm's or under heavy load when the blade also has a high coefficient of lift. When this happens, the pressure on the inlet side of the blade can drop below the vapor pressure of the water, resulting in the formation of a pocket of vapor against the propeller surface, which can no longer effectively transfer force to the water. The air bubbles which make up the vapor are also destructive to the strength of the metal over a relatively brief period of time, causing pitting even in stainless steel.

A propeller VENTILATES when air is introduced to water around a prop, or to the water flow fed to a propeller. This is usually cause by a portion of the propeller blade surfacing beyond it's ability to carry water against the thrusting surface of the propeller during it's travel above the surface of the water. This is the very effect propeller cupping is designed to defeat. Ventilation can also be caused by propeller porting, or by improperly placed or mounted underwater gear such as transducers, pitot tubes and through hull devices.

uncleboo · #6 03-13-2022, 09:36 AM

Thanks for the lesson. I call lots of things by the wrong name.

loon · #7 11-09-2022, 03:28 PM

Quote:

Originally Posted by uncleboo

I have a 13-3/8" x 21P prop on mine and get about 40 mph wot. Anti cav plat about 2-1/4" above keel.

I
I'm surprised at the 2 1/4 height. Do you have a paicture? I have Traders old SF 20 and I believe my plate is about 1" above the keel.