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Discussion Starter · #1 ·
My 1980 Cal 31 is currently equipped with a Universal 16hp inboard diesel engine. Most of the time it is fine, but we have run into situations wherein the horsepower just isn't enough to power through wind and waves in tight quarters where we can't use the sails.

Would like to know what my options are (besides just selling the boat). Can the existing engine be modified to increase horsepower, what replacement engines would best fit my boat and is there a good source for inexpensive rebuilt engines , at what horsepower will I need to increase the prop shaft diameter (currently 1")?

Would appreciate your thoughts and experiences in dealing with this issue.
 

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You have to change your engine and the best option will be a new one instead of rebuilt engine. Shaft diameter is not only dependent on the horsepower of the engine, but also to the revolution of the shaft. If the reduction ratio is higher you will need a thicker shaft.
 

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Discussion Starter · #3 ·
Thank you Celenoglu for the quick response. I found a website awhile back on computing minimum shaft diameters for generators and understand for a given Hp the shaft size will increase with decreasing shaft rpm. However, the calcs were for steel shafts. Mine is brass and not sure how it compares to steel.

I was hoping for a less expensive alternative to increase power 4-8 hp. I think a new engine and labor to replace it will be upwards of $10K.
 

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Maybe look into re propping or re pitching the prop. Maybe that will give you more power? Do you have a 3 bladed prop? If not, they provide more thrust.


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The best prop is the one with one blade which is not possible. The second best is the one with two blades. Increasing the number of blades only help to balance the propellor easly.

Shaft diameter can only be calculated after you choose the engine and reduction gear. You should consider 10K or more for the change.
 

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Discussion Starter · #6 ·
Gladrags1, thinking about it you might be on to something. We have had the boat for only a year and recently motored from Chattanooga, TN to Carrabelle, FL. Best boat speed I could get motoring in calm conditions was 5 mph (4.4 knots) which in my mind is slow even for a sailboat. When we tried to motor against 20 mph wind and 2-3 foot chop, the boat made no progress at all.

I also like the idea of changing props because our current two-blade prop "sings", which is annoying.
 

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Sorry to sound so negative... but there is NO trick or easy way to get 4-6 more hp out of your diesel. Repower. Unviversal makes a brand new 26hp that will fit right in (but it does have 4 mounting points instead of the old 3). And rememer, whatever hp engine you decide on, it's only at that hp when it's at wide open throtle. So... if you have been running your engine at WOT, you've been using 16hp. You want an engine big enough to cruise at 75-80%. You need a 24-26hp engine. I have a 24hp in my Almand 31 and she weighs in at 11,000+... plenty of motor.

Dave
 

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Discussion Starter · #8 ·
Dave E, I don't think your comment was negative at all. If that is the case, it just is. I need to be pulled back to reality occasionally. So, that being the situation the question now is to re-power vs buying another boat that already has a larger engine. The other thing that really hurts is that the existing engine is in perfect condition and we are otherwise very satisfied with the boat.

I wish I could say that picking better wind/wave conditions would be a solution, but that simply isn't realistic when traveling on the water as much as we are.

We appreciate all the input as usual!
 

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I would talk with a prop shop and let them know what boat you have and what engine. The proper sized and pitched prop will make best use of your motor's horsepower. If they are doubtful about better performance then look into repowering. That's just my opinion.


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I am here, fear not.
You can up the power of the engine. Not knowing that exact engine, but you can turn up the fuel on some diesels. Its going to be trial and error. There should be a screw on the fuel pump, to turn it up. Some google work will be needed first. The second issue after you turn it up is cooling.

The 1" shaft should be fine in reality up to a few hundred HP, theoretically, as a 1" shaft should be able to take over 1200FT LBS of torque.

My recommendation to you is to get a larger diameter prop, 4 blade, correctly pitched.
This will give you less slippage, and be more efficient. Try that FIRST, then monkey with fuel settings. You will need to watch your cooling if you up the HP, and if your not blowing black smoke after you up the fuel, you will need a bigger heat exchanger to keep it cool. Expect this to shorten the life of the engine, by half.



OR you could install a 25HP honda outboard and be finished.
 

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I am here, fear not.
You can up the power of the engine. Not knowing that exact engine, but you can turn up the fuel on some diesels. Its going to be trial and error. There should be a screw on the fuel pump, to turn it up. Some google work will be needed first. The second issue after you turn it up is cooling.

The 1" shaft should be fine in reality up to a few hundred HP, theoretically, as a 1" shaft should be able to take over 1200FT LBS of torque.

My recommendation to you is to get a larger diameter prop, 4 blade, correctly pitched.
This will give you less slippage, and be more efficient. Try that FIRST, then monkey with fuel settings. You will need to watch your cooling if you up the HP, and if your not blowing black smoke after you up the fuel, you will need a bigger heat exchanger to keep it cool. Expect this to shorten the life of the engine, by half.



OR you could install a 25HP honda outboard and be finished.
Just noticed your shaft is brass, I still think it should be able to do at least 400ft lbs.
 

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Cal,

If you love the boat, work your way through addressing your power challenge. The upside is that you've already learned her idiosyncrasies, and you've probably already dropped a boat buck or three on fixing all the gripes you've found.

If you buy a new boat, you've got that learning (and spending) curve to go up once again.

If your engine won't achieve rated RPM's in forward at WOT, then you've already got more prop than she can handle. However, if you can get to WOT without the engine straining then you might be able to get a little more power with a different prop &/or pitch.

Repowering is a pretty expensive proposition, and will likely end up being around 2x (or more) the price of the engine. If your current engine is in good shape you may be able to recoup some of the cost by reselling it.
 

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I agree that you are likely under propped. 16hp should be plenty of power for your sub 10k displacement.
The 24hp yanmar was an option on my 10500lb boat and is more engine than it needs. I have a 15/10 two blade folder and never put enough load on the engine to get into the happy zone. I will have more pitch in my next prop.
 

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Discussion Starter · #14 ·
Thank you PorFin. My engine doesn't have a tach., but am reasonably sure it revs all the way up at WOT. I couldn't see any size/pitch markings on the prop when last out of the water so will likely need to get it off and physically check against what a good prop shop would specify. When really pushing the engine, I have used the temperature gage to tell me when I was working it too hard. Normally runs at 185 deg and will climb upwards of 195 deg with throttle full forward. I don't run it long at that temperature so don't know if it would climb even higher.
 

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al brazzi
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Cal,

If you love the boat, work your way through addressing your power challenge. The upside is that you've already learned her idiosyncrasies, and you've probably already dropped a boat buck or three on fixing all the gripes you've found.

If you buy a new boat, you've got that learning (and spending) curve to go up once again.

If your engine won't achieve rated RPM's in forward at WOT, then you've already got more prop than she can handle. However, if you can get to WOT without the engine straining then you might be able to get a little more power with a different prop &/or pitch.

Repowering is a pretty expensive proposition, and will likely end up being around 2x (or more) the price of the engine. If your current engine is in good shape you may be able to recoup some of the cost by reselling it.
Do this.. I have an 8500# 30' boat with a pretty flat bottom I'm sure that helps, but with a three blade fixed prop Its a Motor Sailor I can out run most Sailboats out there. You will not trial a new Boat (candidate) under adverse conditions anyway so I would definitely work on the prop.

Repowering is CRAZY expensive if you don't do it yourself. I wouldn't dream of paying for someone to do that but its beyond most peoples skill set. At that money a newer motor is quite a selling point. Good for the buyer and Good for the owner if you get the first few years yourself.
 

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When you are slogging into wind with the speed control at max what is the engine RPM? What does the data sheet say the HP is at that RPM?

The whacky comment above about one bladed props notwithstanding, trying to get a better prop, three bladed if necessary, is the best idea. Get expert advice from a proper shop.

Fiddling with the injector fuel settings is unwise. Do we assume the designers were fools? The engine life will plummet.

16 hp in a Cal 31 is not unreasonable. I tend to use small engines. Progress is indeed slow in a blow. I've done thousands of miles at 2-3 knots bashing into headwinds and seas.
 
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Fiddling with the injector fuel settings is unwise. Do we assume the designers were fools? .
Some engines are detuned to achieve the HP requirements requested by a builder. This is a very old trick, used in many fields, and works well. Also shortens engine life.
 
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