yamaha 150 4-stroke prop?

[HatTrick]

I just brought my 1986 20 ft home wth a 2005 150 Yamaha 4-stroke. Yeehah! The prop is smaller than the one I had on my old motor. OLD: 1986 merc 150 -15x17. NEW: 2005 150 4-stroke Yamaha -13x19. Does this sound correct? On the Yamaha website they test a 21 ft seaswirl with this motor and use a prop that is 15 1/4x15. Props are all aluminum (I hit a lot of rocks while I fish shallow). What do you experts think? Do I have the right prop?

[Mark]

This isn't the answer you are wanting to hear, but you'd do well to actually try a few sizes out and see how they perform. Although tests are informative as to how the prop worked on a similar boat, even a different 20' SeaCraft may not work well with the same prop that really works great on yours. I found this out when propping my 140 Suzuki: what worked for some of the other SC20's here on the site wouldn't work for me at all...

[HatTrick]

Thanks for the info. I talked to the dealer this morning and I accidentally received a prop for a 115 hp not a 150 hp. He is sending me a 14x19.

[HatTrick]

Well Mark you were right. I am now about to try prop number three. The 14x19 was quick at all rpms but at wot with the motor trimmed I only got a hair over 4800 rpms. It was fast though! With the old '86 2-stroke merc I only ever went 34ish (mph). At 4800 with the 14x19 I went 39.5 on the gps, flat calm, no current, 2 people and 1/2 tank of fuel. I just put on a 14 1/2 x 17 to see if I can get over 5000 rpms. I'll post the results.

[HatTrick]

I tried prop number three yesterday while chasing bass on the Westport River (MA). The latest prop is a 14x17 and this seems to be the one. At WOT and with the motor trimmed I got 5400 rpms and 39 mph. It has plenty of umph to get out of the hole and up on plane. Someday I'll have to get trim tabs if nothing else just to get the nose down at the lower rpms. Attached are some photos of the newly painted transom, new motor etc.. Thanks for the excellent advise, this site is not only addictive but helpful!
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[Briguy]

Nice ride....however. That 150 is sitting low on the transom. You can probably move the motor up 1 or 2 bolt holes. You will probably run 1-2 mph faster. If it were me, I'd move the motor up 2 bolt holes and run the 19" prop. By moving it up you lessen drag and therefore let the prop spin faster. You could probably see 5200rpm's with the 19" or keep the 17" and hit 5800 or so. I don't know what the yam 150's max rpm is. 39 mph seems kind of slow especially when I hear guys run 200's and hit 60mph.

[HatTrick]

Is there a standard for the correct height? I always thought the "fins" on the lower unit were supposed to be at the same level as the bottom of the hull.

[Briguy]

My fins on my flats boat are 1" above the transom. On my 23' they are about 2 1/2" above but I have a 29" setback.

Yes, having the fins at the same level as the bottom of the boat is what the old rule of thumb is but if you have good water pressure and no prop blowout raise it up as high as possible. Steering will be easier, less drag, less draft, better mpg, higher rpm. Boat dealers will almost always mount the motors on the lowest hole. Low liability.

[sailcat]

. 39 mph seems kind of slow especially when I hear guys run 200's and hit 60mph.

Hatrick, Good lookin' ride. I'm running a 200 Johnson with a 15x19 Rapture. Fins are about an inch up on the transom. With 30 gallons of fuel and one person I get about 56 or 57mph on a slack tide with the GPS. I have to use the tabs over 50 to keep the bow down (gets a little light) [img]/images/graemlins/crazy.gif[/img]

[Fr. Frank]

I don't hit 60 mph, but running my nearly 1800 lb 20' Seafari with jack plate, nosecone; jacked and trimmed, I turn my '92 Merc 150 carb at 6000 rpm's and 43 kts (about 50 mph)