Engine and prop too small?

267
90
Sooke, BC
My boat (C&C27 MkII), 5500lbs, 21' waterline, currently has a disfunctional Yanmar 1GM10 hooked up to a 3 blade 12x8 prop.

Assuming I could make the 1GM10 properly functional and putting out somewhere close to the factory power output (9hp, 3600rpm max), is this engine/prop combination still too small do you reckon?

For interest, I ran it through a prop calculator, and to reach a hull speed of around 6.1kts, it'd be wanting 11hp and a 13x10.

An alternative would be to move to a 2GM20 (which I happen to have laying around), which I think would need something like a 15x9.

 
You are in the fuzzy math area of boats.  Your best bet is practical comparisons.  Finding a forum of similar owners and their results with propulsion and prop configuration.  If you do a search here there are active C&C forums out there.  It's always a good standard to look at 80% of a auxiliary engine power curve as max value for intended use.

 
267
90
Sooke, BC
Yeah, I looked in the C&C27 association, and have asked on the C&C mailing list previously. Neither has yielded much if any information. 9hp just "feels" a bit small, but it'll be a lot easier to get a working 1GM10 in there than swapping out to the 2 cylinder.

 

LionessRacing

Super Anarchist
4,286
556
Myrtle Beach,
So many questions: 

  • Do you use your auxiliary to deliver against current, chop and headwinds? 
  • Will your use be a few minutes parking in the slip and clearing the harbor or 3 days passage somewhere? 
  • Do you care about sailing performance? (feathering/folding prop) 
  • Can you generate sufficient charge to keep up with house needs from your engine while motoring somewhere useful in a practical time?  


    100A at 12V eats about 2 HP. 


 

SloopJonB

Super Anarchist
65,127
10,613
Great Wet North
I had a YSM8 with a ?? Martec in a 4300 Lb 1/4 Pounder and it was way more than I needed.

Currently have a 2GMF with a 14X12 Martec in a 6200 Lb 1/2 Tonner and it is also way more than I need.

I'd fix the one you have in it.

 

Moonduster

Super Anarchist
4,823
229
100A at 12V eats about 2 HP

Yeah, no. 100A @ 12V never happens. Let's try 100A at 14.4V, which is 1440W. That's almost exactly 2HP of output power. With heat loss, fan loss, belt loss and a few other factors, efficiency of alternators is around 50%. So to get 1440W out, you're putting in closer to 4HP.

But that one lung diesel is more likely to have a 40A alternator with an automotive regulator and is probably putting out 20A at 13.6 and taking 0.7 hp off the front of the motor, mostly in belt dust.

 
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IStream

Super Anarchist
10,715
2,921
I'd fix the one you have in it.
This.

Even if you were to put the 2GM in the boat, you'd probably want to repair the 1GM before you pulled it so you could sell it or use it as a spare. Might as well fix it first and see how well it does when functioning properly before deciding on whether to swap it out.

 
267
90
Sooke, BC
This.

Even if you were to put the 2GM in the boat, you'd probably want to repair the 1GM before you pulled it so you could sell it or use it as a spare. Might as well fix it first and see how well it does when functioning properly before deciding on whether to swap it out.
I'm in the hole thousands for that fucking little 1GM10 already, I will not spend another cent on it.

I'd rip it out and throw another one in there, it literally couldn't be any worse than the one that is in there, but if I'm going to do that, then might as well go the extra and have some spare power. The 1GM10 is about the smallest possible engine that could push this boat, even then it's not going to achieve hull speed, and the slightest headwind, chop or current and it's going to be challenged.

 
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Phtt

New member
OK, that's new information. If the 1GM is bad then for sure move on. Your 2GM is rated for 13.4Kw at 3600 and 11.8 @ 3400 continuous (from the internet, so it must be right). That will be more power than you need and you will be just pushing a wave if you prop for 3400. See if you can prop for say 3000 and you will be more efficient, less noisy and the engine will last longer, as long as you don't overload it. You will have to be observant and avoid black smoke, which is a sign of overloading, which is very damaging.  The larger engine will also allow you to install a larger alternator if needed. Switching engines will give you the dubious "opportunity" to rebuild all of the gear (prop, shaft, exhaust, hoses, proper gooseneck on the raw water discharge, gooseneck on the exhaust, dripless seal, flex coupling,  add a remote oil filter, reposition the primary fuel filter, clean and paint the engine room, new engine mounts . I am just in the process of rebuilding the gear in my Islander 28 for my very low hour Vetus 2cy that was very shittly installed by the PO reusing the old gear.  Gulp. $$$. But my OCD will thank me when I take the boat into the remote places that I like to lurk.  By the way, the 2GM is a very simple engine (except for the fuel end, that part scares me). I did a compression rebuild myself on my old 2QM in my former boat (rings, cylinder honing, rod bearings, valves and sent the injectors for rebuild).   Not a whole lot different from rebuilding my Triumph motorcycle when I was a kid. Get the shop manual and find a knowledgeable friend. But you will have to buy some special tools (ring compressor, valve spring compressor, torque wrench, micrometer, etc etc.)   Once its in your basement, its a two weekend project and you get to clean up a lot of other sins as well and paint it pink so that it will never fail, it will be too ashamed to be seen by a mechanic.  Good luck. 

 

El Borracho

Sam’s friend
6,247
2,328
Pacific Rim
Your 2GM is rated for 13.4Kw at 3600 and 11.8 @ 3400 continuous (from the internet, so it must be right). That will be more power than you need and you will be just pushing a wave if you prop for 3400. See if you can prop for say 3000 and you will be more efficient, less noisy and the engine will last longer, as long as you don't overload it. You will have to be observant and avoid black smoke, which is a sign of overloading, which is very damaging.
From what I know...this is comment is both right and wrong. If you ‘prop’ for the rated max power at 3400 rpm then it will be also be (magically) correct at 3000. Plus there will be no worry about overloading. Why remove the beneficial, and dearly paid for, option of powering into a gale?

‘Over propping’ can cause overloading at all power settings, not just the higher ones, as it moves the entire load/power curve. Every bit as costly as the torque does the bearing damage, not the power. Black smoke is not a reliable indicator. Being able to reach nearly 3400 rpm in forward gear while securely tied in the slip is a good safe indicator.

 

LionessRacing

Super Anarchist
4,286
556
Myrtle Beach,
100A @ 12V never happens. Let's try 100A at 14.4V, which is 1440W.
The precision of his question, warranted a nominal answer and it's a 12V Nominal spec. 

Trying to pull 100A from a < 10HP engine, bearings are probably the first casualty, but that's another lesson to learn. 

No idea what his house bank is nor loads nor alternator, but your guess of 40A is probably good.

He may not even need one if he's in a slip on shore power and the motor only has to run a few minutes an outing... 

 
267
90
Sooke, BC
Right now it's a stock alternator, which is I think 35A maximum, a larger alternator would be nice as there is no shore power available, but 100A is probably overkill, at the moment I just have a start and a house battery, nothing fancy. Underway, loads shouldn't be silly, chartplotter, VHF, sometimes autopilot. I typically just head out for a few hours, but we get some serious currents, bouncy stuff and winds can pick up quickly. It is a very unforgiving shoreline too, all landings are hard and very pointy, so having grunt to power off a lee shore in 25+kts is a nice thing to have in the back pocket in case my seamanship lets me down.

 

LionessRacing

Super Anarchist
4,286
556
Myrtle Beach,
Part of your (re)power decision should be an analysis of the charging questions above. 

Part of your decision should be the sizing of your cooling system. Mine is sized by Hinckley to run forever at 80% power and < 5 min at wide open. They monitor temp rise to determine when to clean the heat exchanger 

If your budget can stand it a MaxProp allows you to re-pitch in the water

 

hard aground

Super Anarchist
The previous boat (C&C 27 mk1) had a 2GM20 in it. I had the good fortune to pick up a very reasonably priced 12" MaxProp previously enjoyed. Honestly didn't think it was super cramped in the boat. Reasonable enough access to service everything easily enough. 12" was too small of a prop regardless of pitch. Would barely make 6kts at WOT. Do you think there is enough clearance to put a 15" prop on?

 

ryley

Super Anarchist
5,383
583
Boston, MA
Yeah, my budget can't stand a MaxProp, my budget can't really stand this repowering exercise but there you go.
Hey Paul,

your boat weighs about the same as my Columbia which also has the 1GM10. I put a flexofold racing prop on at 14x10 but if I'd gone with the standard it would be a 13x10. I have an 85 amp externally regulated alternator from Ham Ferris, the regulator is a Balmar 614.

The key to happiness for me was adding a toggle switch to my balmar regulator to add "small motor" mode. with my previous prop I could only get 5.9 knots, now I get 7 in flat water. I run in small motor mode until the engine is good and warm but then I can switch it off and get as much out of the engine as possible. You may also need to play with the belt program to reduce the load, but there's no reason your 1gm10 can't push your boat at at least 6 knots.

 

suider

Super Anarchist
You happen to live pretty close (according to your location and screen name-although I suppose BC is bigger than I want to think about) to guys I often use specifically sailboat prop calculations- I have my own stuff that I use for powerboats, but their online calculator is quite good. And, at least y'all speak the same language!  :lol:  But you still need a gear ratio so we can at least know how many turns your shaft is making at the engine's 3600 RPM. 

https://www.vicprop.com/displacement_size.php

 
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