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Just got my new motor (Yamy F150) and mounted it on my newly redone 1974 20ft classic. Lookin for info on the right prop? At full throtle only gettin 5500 rpm and doin 35mph with to guys and a lite boat. Any body runnin a four stroke 150 or similar on a twenty foot seacraft? I am not a speed demon but I thought I maght be able to slide along at 3500-4000rpm atnear 30 mph but only doin 20 at 3500 and 25mph at 4000rpm. Thanks
Capt Russ :D

What's the operating range (rpm) of the engine? How high/low is it mounted? What pitch/dia./material are you running now??

I have a 1975 20 CC, with a 115 Evinrude and I get 37 MPH (GPS) at 5000 RPM. I am raising my motor 1" to get in the 5500 range ( hopefully) I am running a 13.25 X17P prop. I would think with a 150 you should be well over 40 MPH at 5500 unless you are propped way too low. Your hole shot is probably incredible !!

What size wheel are you spinning? I have a 200 yami with a 19" and I'm getting 48 mph, fully loaded 46 gals fuel and two people, don't have the engine to full rpm's yet, althought many people before us say getting to the 50 and over mark is an act of g--, or more money and different lower units. I would think you should be spinning a 19" wheel, Fr Frank, your thoughts?

Just a few things I've learned in my prop search:

1. 4 strokes are heavy - yours is about 170 lbs more than what these transoms relative to the center of gravity were designed for. Keep in mind its also a narrow hull only a 7'6" beam. = Less buoyancy, eh?

2. It seems to me that the most overlooked aspect of correct propping is the shape/geometry of the prop and what it is designed to do. Determine the diameter an pitch of your prop and the name or model number. The latter will tell you what it is designed to do.

3. Check the height of the cavitation plate relative to the keel at neutral trim. Ideal setting seems to be anti-cavitation plate 1/2" to 1-1/2" above the keel. Keel and plate should be parallel when measuring. If the plate is below the keel, that is definitely slowing you down.

4. Not sure what rpms are ideal for your f150 but you're probably close at 5500 so I doubt that's your gremlin. More likely its the attitude of how your boat runs. I'm guessing you probably run bow high and your aft hull is plowing water.

With the extra weight on the transom you almost certainly need a prop with stern-lifting qualities. That's the buzzword for SeaCrafts. If you have trim tabs make sure they're working for you not creating additional drag. If you don't have them, they would help you immensely w/ that 4-stroke. You might also want to consider a foil like a Stingray, Doel-fin or whatever. But your get the same effect with a stout sternlifting 3 blade or probably better in your case a 4 blade prop.

Find out exactly what your prop is first, and share that info. Someone on the site probably has experience with it or knows enough to steer you in the right direction.

Also if you have a baitwell behind your leaning post you might consider moving it toward the pointy end. Same with your batteries if their are situated aft.

One more thing. How did you determine your speed? and if by GPS, make sure settings are reading out mph not knots. If you're reading out knots then your speed is very good.

Good luck.

Since you need about 500 RPM here is a rule of thumb - dropping 2" in pitch should get you 400 and make sure the engine is high enough, you may be dragging extra LU in the water, rasing it will 100 to 200.
I am not familiar with 4 blade props but maybe someone else does, that may be an option as well.

My nephew has a '76 20 that has been completely redone. 2 stroke Yamaha 150 and he hits around 45 depending on passengers and fuel load , with no trim tabs.

I think you need to go to a PowerTech 4 blade stern lifting prop and also raise the motor a notch or 2. I would want the cavitation plate about and an inch or 1 1/2 inche above the "V" on the stern. With the sharp "V", it kind of rules out rule of being even with the cavitation plate. I had a similiar problem with my 23WA and w/FrFrank's advice made the switch and it made all the difference in the world. I have a Merc 4 blade sitting on the shelf for a right hand twist w/Honda hub kit, but Mercury says it is a bow lifiting prop, so that would not be the answer, just the opposite effect. I went from a 14X17 3 blade to a 15 1/4 X 16, 4 blade. check at www.ptprop.com

I would guess that the motor was mounted too low. It makes a HUGE difference. Once you have the right height . . . you can start messing with props.

Quote:

I would guess that the motor was mounted too low. It makes a HUGE difference. Once you have the right height . . . you can start messing with props.

Thats what I was thinking, somethings kinda wrong

fishaholic - Do a search for "hattrick". If I remember correctly he bought a F150 for his 20MA. I think he topped out at 40mph with a F150. I know for a fact that strick's 20ft with an old yamaha 130 two stroke will hit 42+ mph on GPS with 2 people on board.

http://www.classicseacraft.com/forum...r=26413&Forum=,,,,All_Forums,,,,&Words=&Searchpage=3&Limit=25&Ma in=26191&Search=true&where=&Name=1123&daterange=&n ewerval=&newertype=&olderval=&oldertype=&bodyprev= #Post26413

WWp2

Since you have the closest setup perhaps you could post the part number of the yam prop and what bolt setting the motor is mounted on.

Other thing to think about is the transom on the 20 a 20" with a 25" shaft?????? Photo's would help. My best guess at 4000 rpm's you should be doing 25kts/29mph and wot 42 mph minimum.

Brian I have a '78 23cc , the '76 20cc is my nephews who lives in Wilmington,N.C.I live about 180 miles north of there so I can't walk over and check it out . I just sent him an E-mail to see if he could get the info. you spoke about , I will be getting it very soon and will post. He said it is a Stainless prop. and that is all he knows until he gets home and checks further. 5500 rpm's low 40's , trimmed up he can reach 45 but says it starts to get squirrely due to the lack of boat in the water.He is intereted in selling the boat and will forward me pic's and info. on the updates that were made to the boat.It was taken down to the bare hull like some of the boats on the site.I'll post as soon as I get something.I do know that it's a 25" shaft.

Sorry for the delay and I say thank you to all that have given help so far. Let me give you some more info and maby you can help me find the right prop. My boat has a full transom with a fiberglass bracket and jack plate. The motor is about 2-2and a half feet off the transom and mounted one holeup from the bottem witch will put the tip of the lower unit even with the bottem of the hull when jacked all the way up. Swingin a 13 3/4 x17 prop black three blade stainless yamaha. The boat has had all wood cut out, if you want google Hammerhead boatworks and look under rebuilds in progress and the one at the top right is a 1974 potter 20 seacraft and it will answer most of your questions on how it's set up. The only thing different from the picks is my new motor and a t top. Thanks again for all your help to a new member of the seacraft family.
Capt Russ