1982 Cherubini w 12 HP Yanmar - Hull Speed?
- Thread starter polarbear
- Start date
I just took a look at the design as presented on Sailboat Data.
Disp. is 10600 (exactly the same as our 34' boat), but the DWL is 27 feet, and there is considerable IOR-ish tuck up to the stern. No surfing. (!) The speed under power might be more like 6.5 with the referenced size diesel. Not bad.
Pushing more HP into the water will not get you much more speed with that particular hull form. Of course some more HP is nice when you have to motor into a steep chop in a narrow channel.
It's a moderate hull with classic good looks, compared to the current crop of "floating RV's with masts!"
One other bit of YSM12 trivia: if you do not push it to the stop, fuel usage will be about 1/3 to 1/2 gallon per hour. That is a very thrifty engine!
Disp. is 10600 (exactly the same as our 34' boat), but the DWL is 27 feet, and there is considerable IOR-ish tuck up to the stern. No surfing. (!) The speed under power might be more like 6.5 with the referenced size diesel. Not bad.
Pushing more HP into the water will not get you much more speed with that particular hull form. Of course some more HP is nice when you have to motor into a steep chop in a narrow channel.
It's a moderate hull with classic good looks, compared to the current crop of "floating RV's with masts!"
One other bit of YSM12 trivia: if you do not push it to the stop, fuel usage will be about 1/3 to 1/2 gallon per hour. That is a very thrifty engine!
I have a H31 with a 25 ft waterline, hull speed calculated at 6.9 knots, 9700 lb design weight, heavier now.
With the original 2GMF, 13 hp, it would do 6.2 knots in flat water, closer to 4.5 knots against wind and waves.
Currently, I have the 2GM20F, 16 hp, it would do 6.5 in flat water and above 5.5 knots against wind and waves.
Based on the above, you should be doing around 6+ knots in flat water 4+ knots against wind and waves. You should be fine but if the weather is bad and you happen to be in a high current area, you may need to be more careful.
With the original 2GMF, 13 hp, it would do 6.2 knots in flat water, closer to 4.5 knots against wind and waves.
Currently, I have the 2GM20F, 16 hp, it would do 6.5 in flat water and above 5.5 knots against wind and waves.
Based on the above, you should be doing around 6+ knots in flat water 4+ knots against wind and waves. You should be fine but if the weather is bad and you happen to be in a high current area, you may need to be more careful.
My former 33 (1980?) had a 2 QM15. Pushed her at about 6knots in flat water but if I were bucking big waves and strong wind, speed would often drop to barely above steerage. Two bladed fixed prop. Three would have been better. However, rule of thumb indicates 1 hp for every 500 lb.displacement. Therefore, the 12 hp is very undersized, the 15 was barely adequate and something around 22 hp would be so much better. On the plus side of things, the 33 is very easily driven and sails incredibly well, especially to windward ( deeper draft model).
What do you have for a prop? We have a 1990 Hunter 33.5 with a 2GM20 engine and we get 6.5 knots on flat water no wind and 5.5 knots into a 15knot headwind 1-2' waves, yes they are underpowered, later models had a 20hp motor.
The YSM 12 is a big single cylinder, slow reving engine. It produces a LOT of torque. If you compare the specs, the YSM peaks between 3,000 and 3200, the newer higher hp engines peak at 3600 or higher. I would sail the boat and then decide. Torque moves boats, horsepower sells engines.
I have not seen torque specs on the YSM 12, but seat of pants, it is fairly powerful.
YSM ownership is a love /hate relationship. Reliable, powerful, economical but will rattle any loose fillings out of your mouth. They didn't call them YanHammers for nothing. Still and all, after sailing with Thumper for a while, you develop a lot of respect for it. If its a new to you boat, sail it for a while before you start on figuring spending a lot of money to replace a running motor. I'd consider a 3 bladed prop before I jumped to a re-power. Unless you are planning on keeping the boat for longer than forever, the economics of a re-power are against you.
And few experiences equal the transition from power to sail when you shut down a YSM 12!
I have not seen torque specs on the YSM 12, but seat of pants, it is fairly powerful.
YSM ownership is a love /hate relationship. Reliable, powerful, economical but will rattle any loose fillings out of your mouth. They didn't call them YanHammers for nothing. Still and all, after sailing with Thumper for a while, you develop a lot of respect for it. If its a new to you boat, sail it for a while before you start on figuring spending a lot of money to replace a running motor. I'd consider a 3 bladed prop before I jumped to a re-power. Unless you are planning on keeping the boat for longer than forever, the economics of a re-power are against you.
And few experiences equal the transition from power to sail when you shut down a YSM 12!
Our H-33 also has the Yanmar 2GM (fresh water cooled). After a lot of questions we decided to
change the propeller.
This provided unbelievable performance improvement. We went with the 3 blade
sail propeller from Michigan wheel (sailor 3).
I will have to check my notes for the size & pitch etc.
Everything smoothed out and it absolutely increased speed.
All that said, the motor is tiny for this boat but it works very well as aux power.
change the propeller.
This provided unbelievable performance improvement. We went with the 3 blade
sail propeller from Michigan wheel (sailor 3).
I will have to check my notes for the size & pitch etc.
Everything smoothed out and it absolutely increased speed.
All that said, the motor is tiny for this boat but it works very well as aux power.
My Apologies, I had the YSM single cylinder in my head.The 2GM is a newer 2 cylinder design. Newer than the YSM any way. Some of what I said is not relevant to the 2GM, a fine engine but not a torque monster like the YSM. That being said, a 3 blade prop may be a more economical solution than a repower. Sorry for adding to the confusion.
https://spicerparts.com/calculators/horsepower-torque-calculator
https://spicerparts.com/calculators/horsepower-torque-calculator
Interesting conversation I had the same dilemma with my 1982 Hunter 33 with a 2GM 13hp motor, which is some what under powered for some conditions, so to solve the problem without spending a huge amount of funds on repowering with brute force or throwing away a bunch of money on a none power boat (the point of a sailboat is to sail not motor) did the math calculations an came to the conclusion that changing the propeller to a 3 blade and incensing the diameter and changing the pitch was the thing to do... which worked!! wonderfully.. the prop walk is great know .. I have seen a increase in speed at the same rpm of about a knot and half and fuel saving when I do have to motor.. (clean hull helps as well). personal I like the 2GM its simple and easy to maintain KISS (keep it simple stupid) is my moto because things break!!! when your cruseing and the simpler it is the easier its is to fix at sea. (trust me been their done that, got a sticker for it)
I wonder if that would also be the case for YSB 12. Although my engine and prop combination run very well right now, I have little reason to change anything at this point. I'd heard the old YSB was supposed to be noisey and vibrate a lot. Mine does neither. I do have quite a bit of prop walk though.
dj
dj
I've been told by experts that a sailboat pushed with a motor will only go about hull speed or less. If that is your experience then the HP should be fine however I've also been told that more HP on a sailboat helps not with speed necessarily but with resistance i.e. current, wind, waves etc.
I can attest to HP helping with wind and waves. Not so sure about current though as that also is a function of how the hull goes through the water.
dj
It is a function of bottom shape. Newer racing designs are easier to get up on a plane. Flat-ish bottom with less rocker = easier planing. All boats can exceed Hull Speed, it's a question of energy to return that adding more HP will give you. And yes. Big HP coupled with big propeller means better headway against opposing wind and waves.
-Will (Dragonfly)
And few experiences equal the transition from power to sail when you shut down a YSM 12!
Truer words have never been spoken.
BUT you tend to shut down and sail way before the boats with smooth quiet engines do.
Back on topic. If you want to know how fast she will go under power, Fire her up and let her rip.
That is the best way to find out.
Gary
Truer words have never been spoken.
BUT you tend to shut down and sail way before the boats with smooth quiet engines do.
Back on topic. If you want to know how fast she will go under power, Fire her up and let her rip.
That is the best way to find out.
Gary
My experience with a YSM 12, clean boat, two bladed prop, cruising weight, calm sea, and no wind was ~ 4.5 kt. Although I had burst over 5 kt on occasion, 4.5 kt was the number I could most often expect and the best one to use to plan for cruising. The condition of the YSM 12 was very good although the hours unknown.
Cheers, Bob
Hey there !!
Can you share the propeller size and pitch you arrived to after the math? And - empirically -> Now runs smooth,good perf and max RPMs?
Thanks !
Dario
Don't hold your breath waiting for a response. The post is about 8 years old and the poster hasn't been seen in 5 years.
Thanks man. Id need to validate but cant find a recommendation.
Mechanic tells me 14x12
Chat tells me safer to go with 14x10
Do you happen to have the the formulas or calculatior to right size / pitch propeller?
Yanmar 2GM 13hp with KM2-A 2.62 ratio in a hunter 31 1984.
thanks!