Speed - Power - RPM - Pitch Question

A place to exchanges questions and ideas of a technical / procedural nature. Go ahead, try to stomp us !
IanM
Bilge Dweller
Posts: 5
Joined: Tue Oct 20, 2009 8:56 pm

Speed - Power - RPM - Pitch Question

Postby IanM » Tue Oct 20, 2009 9:08 pm

Hi,
I'm new to this forum and have a question regarding the disturbingly low cruising speed of my 34' retired fishing boat. Its a typical westcoast wooden troller built in the 60's, 34', powered by a naturally aspirated Perkins 6-354 (100ish hp) with a borg warner velvet drive transmission. I've measured the crank and shaft rpm using an optical tach, and the resultant reduction is 2.6 : 1, hence cruising at 2000rpm results in a shaft rpm of 770rpm.

The problem is I'm only getting around 5 knots max speed (measured on GPS) and I believe most of these old fish boats typically cruised at 7 - 8 knots. I'm left wondering if I either have the wrong size/pitch of prop or an unsuitable reduction ratio. Given the ratio of 2.6 : 1, what should the size and pitch of the prop be? I don't know what prop I have off hand, but I'll have the boat on the hard this weekend and can check it then.

Thanks in advance :D

User avatar
JK
Enduring Contributor
Posts: 2776
Joined: Tue Sep 04, 2007 2:29 am
Currently located: East Coast, Canada

Re: Speed - Power - RPM - Pitch Question

Postby JK » Wed Oct 21, 2009 3:57 am

I have to ask, is this boat one of the double enders that I have seen around the coast?

User avatar
The Dieselduck
Administrator
Posts: 3012
Joined: Sun Jul 22, 2007 1:41 pm
Currently located: Nanaimo, British Columbia, Canada (West Coast of Canada)

Re: Speed - Power - RPM - Pitch Question

Postby The Dieselduck » Wed Oct 21, 2009 4:20 am

I like Perkins, but ultimately its not all that big of an engine, 100 hp 34 foot wooden boat, I am not surprised that your only getting 5 knots Speed Over Ground (SOG) from your GPS which makes me think right away, where are you operating at and have you factored in current, which of course is a very common occurrence in BC waters. Apart from that, you could measure not only your speed input into the gear, but your output. This will give you the slip, but generally those velvet drives rarely give problems. If your speed is up and the engine is running at full power, then when your up on the ways, have a look at your prop and maybe take over to Rev's or Osborne propeller shop, see what they think.
Martin Leduc
Certified Marine Engineer and Webmaster
Martin's Marine Engineering Page
http://www.dieselduck.net

IanM
Bilge Dweller
Posts: 5
Joined: Tue Oct 20, 2009 8:56 pm

Re: Speed - Power - RPM - Pitch Question

Postby IanM » Wed Oct 21, 2009 9:32 am

JK - its got a squared off transom - not a double ender.

Dieselduck - the boat is mostly operated in deep cove and indian arm, where current certainly can be a factor, but even at slack tide / neutral currents I'm only getting 5 knots SOG. I measured the rpm at the accessory pulleys mounted to the crank at the forward end of the motor, and at the prop shaft. I did this in 500 rpm intervals and got the same ratio each time, so that would indicate that there is no increase in slip as the load increases - I've yet to verify what the nominal reduction ratio is for the model of velvet drive I have, but hopefully its 2.6 : 1 or I have a constant slip problem.

User avatar
alanocean
Leak Patrol
Posts: 34
Joined: Fri May 23, 2008 4:51 am
Currently located: New Brunswick, Canada

Re: Speed - Power - RPM - Pitch Question

Postby alanocean » Thu Oct 22, 2009 3:38 am

Just wondering if there is much marine growth on your hull. you could loose up to 15-25% from drag esspecially on a small boat with low HP in general.

User avatar
JK
Enduring Contributor
Posts: 2776
Joined: Tue Sep 04, 2007 2:29 am
Currently located: East Coast, Canada

Re: Speed - Power - RPM - Pitch Question

Postby JK » Thu Oct 22, 2009 3:51 am

we don't have the double enders in the East, so seeing the motorboats out West was a novelty for me.
I have seen some lovely boats out there.
But, being in ship repair myself, I sooner admire from afar as I am too aware of costs and too cheap to pay for it out of my pocket.

You may find your prop is pitched more for power then for speed. You should check it with a prop manufacturer.

IanM
Bilge Dweller
Posts: 5
Joined: Tue Oct 20, 2009 8:56 pm

Re: Speed - Power - RPM - Pitch Question

Postby IanM » Thu Oct 22, 2009 3:25 pm

There is a year and a half worth of growth on it which is getting scraped this weekend, so we'll see what effect that has.

JK - are you suggesting the prop may have been spec'd for the heavier loads that the boat would have been subjected to when it was operated commercially, sitting much lower in the water? That would make sense, there's probably some truth to that.

User avatar
JK
Enduring Contributor
Posts: 2776
Joined: Tue Sep 04, 2007 2:29 am
Currently located: East Coast, Canada

Re: Speed - Power - RPM - Pitch Question

Postby JK » Fri Oct 23, 2009 2:40 am

That is right, you may need to get it retuned. But, I will tell you, we have played with prop pitch on our small boats and you could spend a fair bit of money and make things worse. If the boat has run with the prop for years, I'd leave well enough alone, unless you have the money to burn.

And you will be amazed at the effect of a years worth of growth on a hull. Take that off and your fuel consumption will drop and speed will go up.
Your money would be better spent on a good anti-foulant and hauling the boat more regularly for an afternoon scrub.

And of course that is only my opinion.

User avatar
Steamboat Al
Leak Patrol
Posts: 34
Joined: Sun Apr 26, 2009 7:01 am
Currently located: Home

Re: Speed - Power - RPM - Pitch Question

Postby Steamboat Al » Fri Oct 23, 2009 8:57 pm

Hi Ian

Why not just go back to the basics. You say you're going to pull her out and get a "Shave & Haircut" so why not have a look at the hub of your prop while your at it, it will have 2 numbers stamped in the first is usaully the diameter of the prop the other is the pitch usually in inches with small props. Multiply that by your rpm (770 in your case) which will give you "inches per minute" so multiply that by 60 for inches per hour. Divide that by 63,360 (inches in a mile) and you will get mph. Now this is only theretical since a prop works in water and your hull is dirty so you will get what is called slip, normal is around 15-20% with small props but you should now have an idea of how fast your boat should go, any big deviation might be caused by the dirty hull if the numbers work out close then your prop is probably spinning to slow. Hope that helps.

Cheers Al

User avatar
The Dieselduck
Administrator
Posts: 3012
Joined: Sun Jul 22, 2007 1:41 pm
Currently located: Nanaimo, British Columbia, Canada (West Coast of Canada)

Re: Speed - Power - RPM - Pitch Question

Postby The Dieselduck » Sun Oct 25, 2009 4:40 am

I learn sometime everyday, thanks Al for the excellent practical answer.
Martin Leduc
Certified Marine Engineer and Webmaster
Martin's Marine Engineering Page
http://www.dieselduck.net

User avatar
JK
Enduring Contributor
Posts: 2776
Joined: Tue Sep 04, 2007 2:29 am
Currently located: East Coast, Canada

Re: Speed - Power - RPM - Pitch Question

Postby JK » Sun Oct 25, 2009 10:20 am

Now that I know Steamboat Al is a Cadet, I feel a little better, he is a whole lot closer to the books then I am. I was thinking, really it is stamped on the props, then I realized I was thinking a mite bigger prop.

Good answer SA and welcome. Keep us old guys honest!

User avatar
Steamboat Al
Leak Patrol
Posts: 34
Joined: Sun Apr 26, 2009 7:01 am
Currently located: Home

Re: Speed - Power - RPM - Pitch Question

Postby Steamboat Al » Sun Oct 25, 2009 12:58 pm

Hi guy's

I hate to tell ya this but "Steamboat Al" is the Chief Engineer and Mike in my cadet so I had him answer the other question for the other fellow about his career as for me, well I've been sailing for 32 years and I started out as an oiler/fireman and worked my way up back when you could do it that way.

Cheers Al :D

User avatar
JK
Enduring Contributor
Posts: 2776
Joined: Tue Sep 04, 2007 2:29 am
Currently located: East Coast, Canada

Re: Speed - Power - RPM - Pitch Question

Postby JK » Sun Oct 25, 2009 2:48 pm

LOL, confuse the old folk.

User avatar
Big Pete
Engineering Mentor
Posts: 846
Joined: Fri Apr 24, 2009 11:18 pm
Currently located: Solihull, England

Re: Speed - Power - RPM - Pitch Question

Postby Big Pete » Tue Nov 03, 2009 2:04 pm

I have been on Holiday, so I came into this late.
Was the Hull clean a sucess?
The only thing I would add is that if you have the Engine makers Handbook or can look up the information for your Make and model on line, I would try to see if you can reach the full rated RPM of the engine and check what the fuel rack position is at that point.
If the engine is unable to reach rated RPM either the engine power is down or the engine is being overloaded by too much pitch on the prop.
If on the other hand the engine easily reaches rated RPM with the fuel rack below maximum then the proppeller has either too little area or too small a pitch to absorb the rated engine power.

As the early discussion mentions, as a working fishing boat, the draft would have been greater and maybe she was set up for towing nets, lines or other boats which would require more power at a lower speed. As a working boat she would have been designed to go to Sea and work in any weather, unless you are a masochist I guess you won't be deliberatly sailing into storms. These factors would also suggest the original designed prop pitch would be much less than the ideal for leisure use.

In theory you could get a higher top speed, reduced Revs (and therefore fuel consumption) at all speeds and the engine would be running with higher torque which is always good. (High cylinder pressure gives better piston ring sealing and high exhaust temps give better combustion keeping the cylinder heads and piston rings clean for longer, thus increasing time between overhauls). However, you must always leave a good safety margin to avoid overloading the engine in bad weather, with a dirty hull and when the engine performance has dropped off a little.

I do not have any practical experience with such small boats, but I would think that with the engine at full RPM the fuel rack should be about 80% on a flat calm day, with the hull clean and the engine developing its rated power. However I would seek confirmation from the prop suppliers on that.

If you go ahead and change the prop, be aware that if you increase the prop diameter, you reduce the clearance between it and the Hull and this may cause severe vibration problems.

Also the prop should have a number of blades that is not directly divisible by the number of engine cylinders, i.e. a six cylinder engine should not have a 6 or 3 bladed prop, this is avoid the prop vibration being synchronised with the engine vibration.

Good luck and I hope you enjoy your time afloat once you have got everything fixed up the way you want it.

B.P.
It is always better to ask a stupid question than to do a stupid thing.

User avatar
JK
Enduring Contributor
Posts: 2776
Joined: Tue Sep 04, 2007 2:29 am
Currently located: East Coast, Canada

Re: Speed - Power - RPM - Pitch Question

Postby JK » Tue Nov 03, 2009 6:09 pm

BP, I hope your holidays included a lot of sun and sandy beaches!


Return to “The Workshop”

Who is online

Users browsing this forum: No registered users and 2 guests