You are not logged in. Please login or register.
The Contessa Corner → Technical → Prop size
I installed a 1gm 2 years ago and run a 12x9 3 blade on my contessa. At 2800 to 3000 rpm I get 5.5 to 6 knots depending on weather. I think the 12 is a bit big for the apature and creates very mild vibration but the boat backs up better than the 10 x13 with the old vire.
I repowered with a Yanmar 1GM10 a few years ago. I used a simple program available on the web that asked question about the boat, engine and transmission then spit out a suggested prop size. Your prop vendor will also be able to make a suggestion based on the data. I ended up with a 11x12 RH 3 blade prop. The cruise speed is about 5.7kt. When I slow down to get better fuel economy (2300rpm, .63L/hr, 4.4kt) I get 620nm on 90L of fuel on board. The engine seems ok with the combination, I have 722 hours on it without any problems.
I have a 26' with a newer Yanmar 1 GM 10 and still have a 2 blade 12x8 prop from the old Ferryman. The previous owner said he had trouble getting over 1 knot when entering the harbor against a 4 knot current. Any advise on a prop for this boat? Thanks, I've not had her in the water, yet..my first boat!!
Ok, the ratio is 2:1 and it says on the prop 12RH9. I don't know what RPMs because it is not yet in the water and I want to change the prop before I put in. Any recommendations on prop?
The engine is rated at 3400 rpm. The calculations are at 94% full power. If you use the 2.21 transmission ratio column and change the 10 pitch to 9, the theoretical speed is 5.36kts. A 10" pitch appears to be ideal resulting in a 5.95 theoretical speed. Your prop can be re-pitched from 9 to 10.
So is this a better choice than another prop? Is this enough knots for this boat? What is the hull speed of the Contessa 26? Thanks alot!
The theoretical hull speed is about 6kts. Get the prop re-pitched and try it out. That will be a lot cheaper than buying a new one.
There seems to be another option offered by my prop consultant..what is your opinion on this? A 3 blade 11x12. Anyone have any experience with this or a suggestion?
PS when the prop guy pulled the prop, he found that the shaft tip was rotten...anyone know the contessa shaft length and size, where to get them and the cost? His charge for a shaft is $300. Seems high. Thanks!
Yes, 300 bucks is high for a 50 dollar peice of stainless bar and 15 minutes worth of machining. But everyone thinks that people with boats have money.... If you went to a "non-marine" machine shop (a shop 50 miles inland....) with your old shaft you'd make out better in the $$ department. Your shaft length will be dependent on the installation in the boat. Best to get measurments off the old one, and if you're still unsure leave 2 or 3 inches on the engine end, and then cut to fit with a grinder and cutoff wheel
Reality may be a bit different:
.875 Dia. #316 centerless ground shafting will cost around $85.00 USD. There are two milling set-ups for machining the two keyslots (one is on the taper). Both slots require the use of carbide tools to speed machining and to get a decent finish. Cutting the taper is slow work. Then comes turning and undercutting for the end thread, threading the sucker, and then drilling the cross hole (after you blasted in the keyway for the prop. If your flange requires setscrew holes, that'll be a few bucks more. Machine time goes for approx $60/hr. My guess is that there is abound 3 hours of shop time at a common machine shop. That's $180 + the cost of materials. Not much room for profit here.
A prop shop is used to this kind of work and they have the equipment to do it well. At $300 there isn't much of an incentive to run it out to Buckey's 'Chinery out in the boonies.
By the way...regardless of who does it, things go better when you give the shop the old shaft and the engine flange so that nothing gets lost in the translation.
my reality isn't that far off, just a bit exxagerated. maybe 15 minutes isn't quite right, but... well, maybe it's just that my home jobs (read boat jobs) take far less time for me to do than customer's jobs. why do i work faster and more efficient if i'm not even gettin payed??
centerless ground is a bit overkill. round bar will work for me, it's just a prop shaft. .875 at a 4 ft length is 65 or so CAD. if you're making lots of shafts it will be chaper....
as far as a "prop shop" being able to make shafts well with the equipment that they've got to do it better - a lathe is a lathe. a mill is a mill. most marina shops have clapped out equipment that came from bucky's 50 miles inland.
as far as the 2 milling setups go, well, that's really only 1. line up a vise, shaft goes in, your Y is along the centerline of the shaft. it's the same whichever end of the shaft you've got in the vise. HSS tools work just fine too, especially in tough materials with small depths of cut. even with hss endmills, 3/4 hour max on the mill for the keyseats. 5 minutes to drill the lockwire hole in the threaded end. the vise is still good to use to, because you havnt't moved your Y which is still in the center of the shaft.
as far as the lathe goes - dial in a bar, cut the taper, cut the major diam for the thread, face the end off, make an undercut with a grooving tool and thread the sucker. with a lathe thats got a quick change tool post, or better yet a turret, some power and is rigid enough to handle a reasonable depth of cut then you're looking at maybe an hour. this is assuming that your lathe man doesn't stop for a coffee break.
this is all assuming, of course, that you're starting from a bare table / machine. a machine shop at a marina makes shafts all day long. more specifically, they setup and run a dozen or more of any given shaft configuration (dia, taper, thread) and leave em real long. so when you need a shaft, all they do is cut it to length. so maybe the first one was worth 300 bucks, but the second and third and the twelfth one that you're getting certainly isn't. maybe there is a bit of room for profit here........
dammit merrill. i'm going to have to stop coming here. all this arguing when i should be working on my boat... cheers
Gawwd, if I'd known that it was so easy, I wouldn't have spent half of a day making mine. I'll send the next one to you. By the way, the shaft house that I know of has two CNC milling centers and three turning centers. Maybe they bought them from Bucky. I'll bet that they winter in Costa Rica.
Half a day is still only 4 hours, thats pretty good if you don't run a machine everyday... But if you do this everyday... Should go quicker. Mitchell time manual pays on the range of 4 hours to do a timing belt in an average car, but most mechanics, and even I can do it quicker. And I don't work on em everyday.
The "marine" machine shops in this area aren't quite as caught up technologically I suppose. Not that you'd really want to set up an nc lathe just to run one shaft. Conversational programming these days make one offs on the NC's pretty easy though.... Might as well do it the old fashioned way and keep the skills sharp. Maybe I should make prop shafts all summer, and head off on the boat for winters...
All I really mean by all of this is that a prop shaft is a pretty simplitic part - requiring minimal machining compared to most of the parts, which are not worth anything near 300 clams each, that I make everyday.
My half day was six hours, from around noon to 6pm. No coffee break but did need a medicinal double scotch around four. Next time, you get the job. Shafting yourself is akin to making your own fried clams.
Soooo, Kristin, what did you end up doing?! ;-) No doubt the guys here have major equipment accessibility, however, I dream of my own lathe among other tools! (it helped being the only girl in shop in Grade 9!!)
Which hull do you have?! Where did you find your boat?! Alaska, wow!
I have a Manecraft dripless shaft seal: very clean and dry. I check the seal regularly, as if it goes, you are hooped! good bye propeller and goodbye boat I have a big fat zinc on the shaft to hold it all on (half arsed at least) if all should fail...
My prop is a two blade 12x9 right hand....does fine for my little 7.5hp Farymann. Remember, in general, the boat will not go faster than her hullspeed/bow wave anyways, so you should just get the best prop for the engine, not the boat, as you want to ensure the engine is working at proper capacity, not the boat being pushed as fast as she can go...does that all make sense?! ;-)
Shannon- thanks for all the advice! I haven't launched yet, went to Asia for a few weeks then brought back a nasty flu..I am stalled on the prop, the prop guy only has the immediate choice of the old 12x9 two bladed to re pitch for the 10hp, or a 3 bladed 11x12 on hand. I am getting that neither are the best choice, so may order another when I figure it out. Sounds like a new shaft is needed too, so will look into what you have.
I found the boat under my kitchen sink when I had a persistant leak and had to call a plumber..with whom i struck up a boat conversation. He had this boat, but had never launched her as he broke his wrist last spring, shortly after purchasing her. So I talked him out of it, to make the story short, and am anxious to start the whole venture. I take a sailing course in Hawaii(the nearest outpost from Alaska) in Feb and hope to have all this resolved by spring! Don't know which hull I have..it is an 1983!
You're welcome...how lucky you were! Good luck on everything..lemme know if you ever want any pics or measurements.
Jim R. who started this thread, what did you decide for your prop? I am sticking to my 2 blade 12x9xRH one...but then I have the original Farymann engine...and I can go 6 knots under power...haven't pushed her much more than that. RPMs are probably close to yours at that point...2,500 anyways.
I really appreciate all the input. I shared this info with my boat-guru Dave Odham of Dave's Marine in Ft. Lauderdale, FL. "Uncle Dave" has probably forgotten more about boats, engines, etc. than I'll ever know and he recommended I go with an 11x9 3 blade. The new prop is in the mail as I write this. I'll install it as soon as it gets here. Only problem is that it's 15 degrees outside and the boat is buried under a foot of snow. I'm afraid we have to wait until spring to see the outcome.
You mean 15 degrees Fahrenheit?! hee hee, 15 degrees up here in Canada (Celsius) is mighty fine! ;-)
But I did see some snow a few weeks ago, not fun!
Let us know how the prop works!! Nice thing with our rudders is there is room to take prop off without taking off the rudder.
Still struggling with the prop issue. The choices are: the original 12x9 two blade but repitching it 1", a 11x12 3 blade which is on hand, or what other options have worked to get the RPMs up to 3400 on anyone elses Yanmar 1gm 10hp? Help!! I must decide the best choice as it is the last chance before launch and I dont' want to have to take her out again for a while...ps...we have strong currents here...
Jim- great that you found the prop! Where did you get it please, and for what price? Do you have strong currents there? Does anyone have advise regarding feathering props?
I would recomend a 3 blade prop if u want more control while reversing or docking. The 3 blade prop will also give you more bite while powering. You will have a slight performance lose while sailing but it is not a race boat so i wouldn't be too worried about it. I have a 3 blader on my 1gm10 and it does good work, my boat is out of the water but unfortunately it is about 1000 miles away from where i am now so i am unable to measure the pitch. I sail on the coast of British Columbia with strong currents and my average cruising speed is approx. 5.2 knots so it is not very nice to deal with 2-3 knots of current all the time.
Hi, I have a 3 blade prop on my Yanmar 2GM20; I'll measure it at the weekend.
Sailing around the Irish Sea and Anglesey we get some pretty strong 5knot+ tides which makes for good practice on the old tidal calculations and avoiding wind over tide conditions.
Just got the engine back in this week so hope to get her back in the water soon and find find out what the cruising speed is with a clean bottom and prop !
Cheers
The Contessa Corner → Technical → Prop size
Powered by PunBB, supported by Informer Technologies, Inc.