How Fast Does Your Boat Motor?

[thomm225]

Originally Posted by TooCoys

I don't have a tach. I've been looking for instructions on how to install one, but I'm coming up empty handed. If I can't figure out how to install it on my own, it will be a few months before I can continue to diagnose the problem.

You shouldn't need a tach.....just a bit of experience.

Take the sails down and see how it motors in calm water with no current at about half throttle or in current but try motoring with and against the tide to see the difference.

Stay close to land so you can see your progress

I rarely run my 5 hp outboard over half throttle and can still get 5 knots motoring speed. My boat displaces 6600 lbs

[zedpassway]

You should be able to get to hull speed on your motor. I can go 5.5 knots on 1/4 throttle using my 6 hp Tohatsu in my 19 foot sailboat, against a 15 knot headwind and a 3 knot tide.

You have a problem.

[hamburking]

I've owned several boats in that size range, including 2 different C&C27s (mk2 and mk4). They easily motored up to 4 knots, but it took a lot more revs to get to 5 knots. I knew guys who just always use WOT and enjoyed 6 knots and lots of engine repairs.

This past summer I delivered a Mirage 27 with a yanmar 1gm (with a loose cutlass bearing). Easily motored up to 5 knots, and when pressed made close to 6.

If you leave the engine in neutral while sailing, the prop will start to spin, and make a little spinning sound easily heard below, at around 5 knots. Its easy enough to look at the prop shaft from inside one of the cockpit lockers on most boats.

It could be a clutch problem as mentioned in another reply. This is also easy to check. When you put the engine in gear, it should have a nice solid THUNK sound. If you watch the shaft, it should instantly be turning at speed. If there is no thunk, or if the shaft slowly accelerates, then the clutch(s) are slipping.

The most likely cause of the trouble you describe is that you just aren't revving high enough. Give her some gas! Do you have a tachometer? You want to be around 2000 rpm for a small diesel like yours.

[thomm225]

Originally Posted by zedpassway

You have a problem.

Does he have a problem, or was he sailing/motoring against a strong current?

I used to watch the monohulls sit dead still in Pensacola Pass trying to get out through a strong current

I was on one of my beach cats either sailing right by them or sailing with boards up over in the shallow water (if racing out against the current) where the current was much less

[Kelkara]

My boat (27ft 7000 lbs) will comfortably do a little over 4 knots at 2500 rpm (Beta 16) ... I haven't really pushed it much harder than that ... I've had it sailing at 5 knots, but my sail trim needs work ... I'm due a haul-out but don't have any obvious growth.

[Suijin]

Yeah you have to know what kind of current you're in. If you don't have access to that information from GPS or another source, look at water around pilings, buoys or any fixed object in the water for reference. If they are leaving a wake, you've got a decent current. Current can be deceptively strong in places and you won't know how strong unless you have a fixed point of reference or can compare speed through the water to speed over ground.

As someone else asked, what RPM were you turning? If your tach is not working, you may not have been running as high as you thought you were.

Vibration could be a number of things, alone or in combination; bad cutlass bearing, bent shaft, loose/poorly alighned coupling, engine alignment, transmission problems, etc. That said, sometimes a boat will vibrate more at certain RPM's due to harmonic vibration; if you change RPM and the vibration varies and goes away at times, it's probably harmonic.

Your prop could also be fouled with a rope or something else, which would cause both vibration and low speed.

[Teknishn]

TooCoys, FWIW, I have a video from the PO of my '81 Cherubini 27 in a calm channel motoring along at about 4-knots in March of last year (Note: I haven't seen my boat yet as I will be repatriating to the US in about a month). Marine survey of the boat prior to purchase states a speed of 5-knots was attained during the survey in the same channel at slack tide. I also have a 1GM coupled to a Kanzaki gearbox.

The Hunter Cherubini's were notoriously under powered. This, according to posts on forums both here and elsewhere, and my research into the boat prior to purchasing.

TrentePied's post on distance and counting elephants is spot on (but I'll disagree with the part about not needing instruments just because I work on boats/yachts for a living and am a bit anal about monitoring systems....but that's just me). TrentePied is correct that you don't need a whole bevy of gauges installed on the boat for coastal cruising...just a good compass, binoculars, and knowing how to navigate are enough on this size of boat.

As seabreez suggested, go run the boat between two buoys on a calm day (or two other known points...GPS will work here as well). I will add here to do it in one direction, and then the other....then do it again, and one more time just to be sure. Average them and then I think you will start understanding the performance of your new-to-you boat while underway with/against current.

Wish you the best of luck with "KaySea" and I look forward to hearing more about you adventures as I prepare to start mine in the not too distant future.

[akprb]

What newhaul said.

3 blade, take the time to pitch it properly.

[mickelsen]

The prop is a problem and needs to be replaced also the same for the cutlas bearing. I am concerned that you would run her with a deteriorated bearing as it will no kidding give you some vibration. The bearing is made of rubber which has now become too hard and is going elliptical on you which makes the shaft move around. Think blade surface area which delivers thrust if you have the correct prop...
Don't ignore the damage which may be done as the shaft sends the vibration to the transfer gear.

[mickelsen]

Adding to my previous the barnacles will disrupt the the flow as it goes thru the water column creating cavation which only allows the prop to spin in it's own turbulence.

[ctl411]

I had that engine in a Cherubini 30. It is not going to get you hull speed without full throttle clean bottom clean prop. Even then maybe not with a 2 blade ever. That said I would not put a 3 blade on it. It's a sailboat and in decent wind will sail faster than it can motor. So unless you have to deal with channel current a lot stick with a 2 blade. As for speed 4 to 4.5 knots at 3/4 throttle. Also like others have said very little growth on the prop will kill performance. You don't have much extra horsepower or blade area to overcome a inefficient prop. I've been dealing with the same problem on my G50. Two blade eBay Max Prop not enough blade area. If I get just slime on it performance drops drastically. I put up with it because I gain 3/4 knot under sail plus or minus.

[akprb]

Odds are your shaft is work hardened as well. Consider replacing while your at it.

[Sailmonkey]

Originally Posted by akprb

Odds are your shaft is work hardened as well. Consider replacing while your at it.

Huh....from a worn cutlass? Seems a little excessive.

[newhaul]

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[TrentePieds]

TooCoys:

I forgot my ABCs: A= Assume nothing. B = Believe nothing. C = Check everything :-)

It bugged me all afternoon that the 11" prop I recommended just "felt" too small. So I checked the reduction ratio on your tranny ("clutch" is the more conventional appellation). I had assumed that it would be 1:2, which is pretty normal, i.e. two turns of the engine to turn the screw one revolution. In fact the RR is 1:2.64, i.e. 2.64 turns of the engine are required to turn the screw once. That makes quite a difference.

Given this "new" knowledge, the recommended prop is a 3-bladed right-hander of 12" diameter and 8" pitch. If for some reason you want to use a 2-blader it should be 13"D x 8"P

Looking at your pictures, the existing prop seems to be pretty close to that. Your engine should be able to turn 3,000 to 3,200 RPM at WOT. If the prop's diameter is greater than the numbers I've given you, the engine will not be able to reach such RPM. It might not go higher that 2K or thereabouts, and the prop cannot therefore "screw itself into the water", in any given amount of time, as far it could at higher RPM. That means that your boat cannot go as fast as it would with the proper prop.

Let's know how you make out :-)

TrentePieds