Boat speed and rpms

[Douglasdzaster]

Hello everyone,
I have a 1982 , 1652 Monark with a 1992 Yamaha 40hp. 3 cylinder, 2 stroke tiller. The boat itself shows to weigh 274 pounds according to the specs I’ve found.
How fast should I be going with just myself ( 260 pounds), one battery, and 6 gallon fuel tank? So far I’m seeing 30 mph according to my phone’s gps. It has a quick hole shot and trims out great. I’m own my third little tachometer trying to find an accurate one. It shows idle around 800 rpms and the boat shifts smooth drops to 600rpms or so in gear and idles out of the cove fine. But wot it only shows 2500 to 2700 rpms. Thinking it’s reading half rpms. I’ve got a different prop the pushes to boat a lot better at all rpms. But wot I hit 3,000 to 33,00 rpms. So I stay out of it. Doesn’t sound like I’m pushing the engine to hard but I’m not going to trust my ears and blow my engine. I was hoping to hit 40mph or so.

 

[DaleH]

Boat Speed and Prop Slip calculators, link = https://forum.tinboats.net/viewtopic.php?f=4&t=42747

Run your rig through this, link = https://forum.tinboats.net/viewtopic.php?f=4&t=37433

 

[Zum]

What pitch prop are you using? I figure if your in the 14/15 range, you shouldnt have to worry about reving to high but a tach would be best. I had a 40 2 stroke on a 1652, little heavier than yours (370ish) and would get approx 33mph, if Im remembering correctly.

 

[Douglasdzaster]

The one that runs the best is 10 1/8” x 15 pitch. The one running at lower rpms is 10x16. The first one will hit around 34-35 mph. But it may be over rev. If my tachometer is showing exactly half. It seems accurate at idle. Shows 750-800 rpms and goes into gear smooth. But wot it seems to read half. The 16 pitch 2,500 - 2,700 rpms and 3,000 to 3,300 with the 16 pitch. I’ve done small engine repair for a long time but this is my first outboard. I can listen to a chainsaw or trimmer and tell when it over revs. The outboard sounds good to me but I don’t want to chance it. I’m on my third tachometer wondering if they make a small one that’s accurate. I’m using the ones that you wrap a wire around a spark plug wire.

 

[Zum]

I bought the tiny tach brand and it worked perfect,lasted 5/6 years. I cheaped out and bought a knock off this time and it seems to be double of what it's supposed to be. I dont know maybe I just dont have it.set right. The tiny tach people had good customer service and will tell you which one you would need. Also go online or here, theres probably someone that could point you in the right direction.

 

[gogittum]

I'm having a very similar problem and started with a cheapie induction tach - with the wire wrapping around a spark plug wire. It cut in and out and readout jumped all over the place. It lives out at the land-fill now. Seat of my pants told me I was only turning around 4,000 rpm. (which turned out to be a bit low) Finally broke down and bought a TinyTach last week, on rec' of many others. I didn't want to spend ~$150 + on a "real" tach for a simple tiller tin skiff.

Yes, it was 50 bucks and the other was ~$25, but the TinyTach gives a good, solid reading with 1 1/2 wraps on my Suzuki DF25 V-twin. Now I've got something to work with and the same questions. Going with the articles above, they give a much higher pitch recommendation than what works for me. (the rec'ed 14" pitch only gives me 25.7 mph @ 4,600 rpm; the 13 pitch gives 25.4 @ 4,900 ??? Max rpm for the motor is 5,500) Not sure where to turn from here and can't afford to just shotgun props at the thing.

When I bought the motor, I'd hoped for 30 mph and cruise of 25. Doesn't look like I'm going to see it.

 

[old_fart]

I'm having a very similar problem and started with a cheapie induction tach - with the wire wrapping around a spark plug wire.

Not hard-core enough...

http://n952.ooguy.com/tach/#TACHOMETER

(There's a 2021 Merc 25 on the boat from which two of the images were taken)

 

[Zum]

I'm having a very similar problem and started with a cheapie induction tach - with the wire wrapping around a spark plug wire. It cut in and out and readout jumped all over the place. It lives out at the land-fill now. Seat of my pants told me I was only turning around 4,000 rpm. (which turned out to be a bit low) Finally broke down and bought a TinyTach last week, on rec' of many others. I didn't want to spend ~$150 + on a "real" tach for a simple tiller tin skiff.

Yes, it was 50 bucks and the other was ~$25, but the TinyTach gives a good, solid reading with 1 1/2 wraps on my Suzuki DF25 V-twin. Now I've got something to work with and the same questions. Going with the articles above, they give a much higher pitch recommendation than what works for me. (the rec'ed 14" pitch only gives me 25.7 mph @ 4,600 rpm; the 13 pitch gives 25.4 @ 4,900 ??? Max rpm for the motor is 5,500) Not sure where to turn from here and can't afford to just shotgun props at the thing.

When I bought the motor, I'd hoped for 30 mph and cruise of 25. Doesn't look like I'm going to see it.

Not sure what you have for a boat but like you say those speeds may not happen(lots of variables). WOT for me is around 28mph...30hp 2 stroke on a 1652

 

[maintenanceguy]

My gut says you be able to get over 30mph, but just barely. Which prop you have makes a big difference. If you have trim tabs or a hydrofoil, knock off two or three mph because you're using up some of the power to lift the stern instead of go fast.

 

[gogittum]

I'm having a very similar problem and started with a cheapie induction tach - with the wire wrapping around a spark plug wire. It cut in and out and readout jumped all over the place. It lives out at the land-fill now. Seat of my pants told me I was only turning around 4,000 rpm. (which turned out to be a bit low) Finally broke down and bought a TinyTach last week, on rec' of many others. I didn't want to spend ~$150 + on a "real" tach for a simple tiller tin skiff.

Yes, it was 50 bucks and the other was ~$25, but the TinyTach gives a good, solid reading with 1 1/2 wraps on my Suzuki DF25 V-twin. Now I've got something to work with and the same questions. Going with the articles above, they give a much higher pitch recommendation than what works for me. (the rec'ed 14" pitch only gives me 25.7 mph @ 4,600 rpm; the 13 pitch gives 25.4 @ 4,900 ??? Max rpm for the motor is 5,500) Not sure where to turn from here and can't afford to just shotgun props at the thing.

When I bought the motor, I'd hoped for 30 mph and cruise of 25. Doesn't look like I'm going to see it.

Not sure what you have for a boat but like you say those speeds may not happen(lots of variables). WOT for me is around 28mph...30hp 2 stroke on a 1652

Boat is a Starcraft Seafarer; 1663. Fully loaded weight in the water with me and dog is about 700#.