1964 Speeditwin 28hp / WOT RPM - Prop sizing

[kdiddle]

Good morning all. I have a quick question that I'm hoping the Evinrude gearheads in this group might be able to help me with.

I have a small 14ft Crosby Sled running a 1964 Evinrude Speeditwin that I've go through (bored .040 over, new internals/carb/etc). It runs like an absolute dream both at low/high speeds, but I have a question. The original 1964 prop hub finally spun last year so I replaced it with an identically pitched prop from Tacoma (9 1/4 x 7), but now that has me questioning whether the original prop was proper.

Here is my question - despite the engine running super well, it does seem to struggle a bit to get going (keeping in mind the boat and the engine are both nearly 60 years old), so I put a simple digital tachometer (one that you wrap around spark plug wire) and its showing that my WOT RPM when lightly loaded is about 4200 RPM. The manual for this engine says its rated at 5500 - if this were your boat, would you attempt to find a lower pitch prop to get WOT closer to 5500?

 

[kdiddle]

Well, crap...I suppose this means you can disregard my question above. I just spent some time trying to find a prop with a flatter pitch than 7 and I'm coming up blank. So it seems it doesn't matter that I can't reach top RPM, it appears I can't buy a prop any lower geared than the one I currently have...

 

[kbait]

Move tilt pin toward transom/redistribute weight toward front of boat to plane quicker.. what is your top speed after these adjustments?

 

[kdiddle]

Thanks Kbait. I have played with the tilt pin both up and down and ended up moving my trolling motor to the bow (along with the super heavy deep cycle battery) up front put more weight on the front. It does tend to ride pretty tail heavy with just my 200lb self in the back.

With its current set up, it runs 22mph at WOT (approx 4200 rpm). Believe me, it feels like 122 mph in this little rig. So I'm not concerned about going faster, I was more concerned with making the engine work too hard on planing out.

 

[F_R]

There is something wrong with this story. That motor should be screaming with such a small prop on such a small boat. That prop should be ok on a houseboat or something.

 

[racerone]

Correct-----Motor should be screaming with 7" pitch prop.----Post pictures of this boat.----I say put a 10 or 11" pitch on it.

 

[kdiddle]

Sorry don't have a pic handy except on a previous post of mine (Post #3) - https://forums.iboats.com/threads/1964-crosby-28hp-speeditwin-rescue.705093/

Is it possible the boat is just heavy? It is fiberglass, but i did singlehandedly walk it backwards up an uphill driveway all by myself, so i know it can't be that heavy... I still have the original prop, and I can't find a stamp on it anywhere that indicates a part # or pitch.

 

[kdiddle]

Annnnnd now I'm questioning everything, since it seems 7" pitch should be way too low for a boat this small...I suppose I could post a video of it running and planing out to see if it sounds/looks normal to you guys...

 

[F_R]

I don't actually remember what a Crosby Sea Sled is. Does it resemble a Boston Whaler?

 

[kdiddle]

Hey F_R, its actually just a small fiberglass jon boat. I linked my post above that has multiple pictures of it

 

[kdiddle]

Wait, wait, wait....before I go blowing up my sweet running motor...I'm beginning to doubt my little wrap on tachometer...How are you guys measuring rpm on these old motors??? When WOT it sounds like its absolutely screaming....so now I'm thinking my tach is lying and the only reason its only running 22mph is because the prop is too low/slow...

 

[racerone]

Boat should run about 30 mph with 1 person and 11" pitch.

 

[kdiddle]

Well there you go, guess I need to throw my tach in the trash (its literally crap from you know where overseas) and maybe grab a spare 10" prop (not that I want/need to go faster than it does already).

Thankfully I haven't been running it full throttle at cruise since it seems to make a lot more noise for not much more speed ( and also chugs way less gas)...guess I know why now...

I'm still very curious what the best way to measure rpm on an engine this old while running down the lake. Please share if you have any experience in that department. Thanks again all!

 

[racerone]

Many people use a ---smart phone------ and reflective tape to measure RPM of almost anything that rotates.

 

[F_R]

OK, I looked at the link, and that's what I was thinking when I suggested it might resemble a Boston Whaler. That thing should be just about scarey with that motor. You are way, way, under-propped. It might take as much as a 12". Could your tach be showing 1/2 actual RPM? I dunno, never heard a 28 running at 8400 RPM.

 

[racerone]

If it runs at 8400 RPM it should be revered / donated to a university for scientific research.----It be one of a kind.

 

[kdiddle]

Haha @racerone! I def think it's turning less than that...but get this... I recall my parents telling me my grandparents used to go fishing in that boat WITH another couple (apparently people were smaller in the 70s)and they also said they saw him pull a skier on more than one occasion when they were kids...so I'm guessing my grandpa opted for the 'torquey' 7" pitch for such shenanigans... Haha

Not sure if this will work, but I found an old video of her running out at WOT for some context on engine speed/sound...https://youtu.be/0n45NMpRagU

 

[kdiddle]

That link didn't work - let me try again:

 

[oldboat1]

Factory prop was aluminum 10 3/4 x 11.

 

[kdiddle]

So I might need to go post this in the prop forum, but I looked at iboats.com and they don't carry any pin drive props that would fit my engine. Looking at Tacoma props, the closest they have is a 9 x 11. Since that would be a smaller diameter prop, does that affect performance compared to the stock 10 3/4 prop?