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WannaB
02-15-2010, 10:01 PM
Hello,

We have an 8 ton full keel sloop. 25" lwl with a 10' beam and 4'6" draft. I have a chance to buy a Volvo 28 hp with 2.4 gear. But, I am limited to a 14" prop if I maintain a 10-15% clearance. I realise a 3200 rpm motor would be better served with a less than 2.00 gear and this small a prop.

The Volvo is new and priced very nicely is my reason for consideration. But, the gear leaves me to wonder if I should keep looking? What would those with more experience than I offer for suggestions?

Thanks for your time.

JimConlin
02-15-2010, 10:26 PM
Dave Gerr's propeller book or a good prop shop can answer your question.

Todd D
02-15-2010, 10:37 PM
A run of your data through my prop calculator (based on Dave Gerr's book) suggests that 28 hp won't give you more than about 6.2 knots with allowances for power loss in the trans and to run the alternator. Given that a 12" pitch prop should be fine. I would suggest a 3 blade prop for that diameter. You don't have enough power to take advantage of a 2:1 gear unless you go with a 10" pitch or less. If you are going to use a 2 blade prop, I would argue for a 10" pitch prop and the 2:1 gear. With a 3 blade the 2.4:1 ratio with a 12 inch pitch should be about right.

wizbang 13
02-15-2010, 10:41 PM
2.4 gear is good for 3200 engine. ideal shaft speed is aprox 1000 . propeller graphs use shaft speed and hp, not engine rpm. 14' is close. sawzall? (boat not prop)

Todd D
02-15-2010, 10:45 PM
Actually, given your displacement, I think 28 hp is a bit minimal. Your hull speed is 6.7 knots using 1.34 x sqrt(LWL). To cruise at 6 knots you need about 24 hp at the prop. Assuming that is 80% of full power, you need about 30 hp and preferably 35 hp to allow you to make hull speed. In addition, the 28 hp engine likely doesn't have enough torque to push the boat much above 6 knots unless you go to a higher gear ratio than 2.4.

wizbang 13
02-15-2010, 10:57 PM
some would say 28hp is 3 times what you need for 8 tons. 5 knots is a more reasonable speed for 8 tons @25 ft.

Breakaway
02-15-2010, 11:09 PM
You could consider a four-bladed prop, which would increase your disc area ratio and your blade area for the same diameter, so as to better absorb the power (that's your question as I understand it, yes?)

WannaB
02-16-2010, 09:35 AM
A run of your data through my prop calculator (based on Dave Gerr's book) suggests that 28 hp won't give you more than about 6.2 knots with allowances for power loss in the trans and to run the alternator. Given that a 12" pitch prop should be fine. I would suggest a 3 blade prop for that diameter. You don't have enough power to take advantage of a 2:1 gear unless you go with a 10" pitch or less. If you are going to use a 2 blade prop, I would argue for a 10" pitch prop and the 2:1 gear. With a 3 blade the 2.4:1 ratio with a 12 inch pitch should be about right.


I have no problem buying a good book. So I have added Dave's to my list. Thank you for sharing the prop info. Yes, I was planning on a 3 blade and 12" pitch is about what I expected from my searching.

WannaB
02-16-2010, 09:49 AM
2.4 gear is good for 3200 engine. ideal shaft speed is aprox 1000 . propeller graphs use shaft speed and hp, not engine rpm. 14' is close. sawzall? (boat not prop)

That is why I provided the engine rpm so shaft speed would be available.

The Kubota based engines I have checked into use 3600 as max rpm and the dealers suggested roughly a 1.5 gear and a 14x8 prop.

Sawzall isn't a consideration at the moment. But it is a popular and useful tool for many projects.

Dale Genther
02-16-2010, 10:09 AM
My prop. calculator show that, at a max. RPM of 3200 the 28 hp will give you 6.5 knots with a 14 in. diam. X 12.2 in. pitch 3 blade prop. At a cruising rpm of 2720 (85% of max. rpm) you will get 5.5 knots with the same prop. All of which is right in line with what my boat, Radiance, does: 26 LWL, 7.5 tons, 24 hp at 3400 max. continuous rpm. Prop 13 in X 13 in 3 blade.

WannaB
02-16-2010, 10:18 AM
You could consider a four-bladed prop, which would increase your disc area ratio and your blade area for the same diameter, so as to better absorb the power (that's your question as I understand it, yes?)

Yes, getting the power transmitted with the prop limitation of 14" and in this case a low gear. As I mentioned above, dealers had suggested a 1.5 range gear for a 14x8 prop with similar hp @3600.

Some here have suggested more hp to reach theoretical hull speed. Even the dealers I've spoken to are not trying to sell more hp due to the limit of a 14" prop. I'll go ahead and contact a prop shop and see what they say as someone suggested.

A 6 knot cruise is perfectly acceptable to us.

I actually wondered if anyone by chance had a boat similarly configured and how it worked for them.

A new Kubota based engine was what I was planning and this opportunity came up so I thought I'd ask some opinions.

Thanks for all your responses.

WannaB
02-16-2010, 10:41 AM
My prop. calculator show that, at a max. RPM of 3200 the 28 hp will give you 6.5 knots with a 14 in. diam. X 12.2 in. pitch 3 blade prop. At a cruising rpm of 2720 (85% of max. rpm) you will get 5.5 knots with the same prop. All of which is right in line with what my boat, Radiance, does: 26 LWL, 7.5 tons, 24 hp at 3400 max. continuous rpm. Prop 13 in X 13 in 3 blade.

Dale,

I had hoped someone with similar criteria would pop in to share info. Sounds like acceptable performance for our boat also.

I need to decide in the next few days on the engine as the seller wishes to see it have a new owner soon.

Thanks for your reply.

Todd D
02-16-2010, 01:44 PM
The power formula in Dave Gerr's prop book says you will need 29 hp at the prop to do 6.5 knots and 23 hp at the prop to do 6 knots. A nominal 28 hp engine will have about a 3% power loss in the gear box and will also lose up to 1 hp to run a 50 amp alternator. That means that 23 hp at the prop requires an engine output of 1.03 x 23 + 1 = 24.7 hp. Assuming you are looking at a Volvo D1-30 (formerly 2030), the engine achieves 24.7 hp at about 2550 rpm with a torque of 51 ft-lb. You can achieve that with a 13.6" pitch prop which should need about 47 ft-lbs of torque to generate the needed power at the prop. That seems to be within the capabilities of the engine. I note that the actual gear ratio available with the Volvo D1-30 is 2.35:1. However, to the extent that Gerr's power formula is accurate, you will hit the power wall at about 6.3 knots at about 2,600 rpms. I notice that the torque curve for the D1-30 drops off from a maximum of 53 ft-lbs at 2,200 rpm to 46 ft-lbs at 3,200 rpms. Torque required at the prop to generate the power needed for 6.3 knots is about 53 ft-lbs, which isn't available at the rpms required. Consequently, I think you are likely looking at more like a 5 knot cruise speed with that engine and gear. Volvo also lists a 2.63:1 transmission as available with that engine. I think that would be a better choice since it makes more torque available at the prop. With the 2.63:1 gear and a 13" pitch prop you should be able to achieve maximum engine rpms at about 6.3 knots.

All of the above is predicated on the accuracy of Gerr's formulae in his prop book and will require ground truthing.

wizbang 13
02-16-2010, 07:37 PM
I use a md2040 on my 12ton, 30 'wl. PLENTY of power. Cruise @ 5 @ 2100rpm. 6 1/2 K is wasting fuel.

wizbang 13
02-16-2010, 07:39 PM
These transmissions haved different ratios fwd or rev .whats up with that? One for displ. and one for lighter weight??

Todd D
02-16-2010, 08:31 PM
Power required to push a displacement at displacement speeds (less than about 1.4-1.5 time square root of waterline length) is a function of waterline length and displacement. The power requirement is directly proportional to displacement but is exponentially related to the LWL. Consequently, the power requirement goes up quite quickly as you approach 1.34 times the square root of the waterline length. For example Gerr's formula gives the prop horsepower required to push Wizbangs 12 ton 30' LWL boat at 6 knots as 26 hp. Similarly, WannaB's 24' LWL 16,000 lb displacement boat would require 23 hp to make 6 knots. The speed relative to nominal hull speed is a much bigger player in determining required horsepower than is the displacement.

Similarly, Wizbangs 38 hp Volvo 2040 has plenty of power to push his boat up to about 6.7 knots. The same engine would only be able to push WannaB's much smaller and lighter boat to about 7.1 knots largely because of the smaller LWL.

Gerr's formula relating power required (hp) to boat speed' displacement and waterline length is:

hp = disp/(10.665/sl)^3 - (disp = displacement in pounds, sl = speed in knots divided by the square rool of the waterlin length in feet.)

I think transmission generally have lower ratios in reverse because props tend to be less efficient in reverse. Thus higher prop rpms are desireable.

WannaB
02-17-2010, 10:00 AM
The power formula in Dave Gerr's prop book says you will need 29 hp at the prop to do 6.5 knots and 23 hp at the prop to do 6 knots. A nominal 28 hp engine will have about a 3% power loss in the gear box and will also lose up to 1 hp to run a 50 amp alternator. That means that 23 hp at the prop requires an engine output of 1.03 x 23 + 1 = 24.7 hp. Assuming you are looking at a Volvo D1-30 (formerly 2030), the engine achieves 24.7 hp at about 2550 rpm with a torque of 51 ft-lb. You can achieve that with a 13.6" pitch prop which should need about 47 ft-lbs of torque to generate the needed power at the prop. That seems to be within the capabilities of the engine. I note that the actual gear ratio available with the Volvo D1-30 is 2.35:1. However, to the extent that Gerr's power formula is accurate, you will hit the power wall at about 6.3 knots at about 2,600 rpms. I notice that the torque curve for the D1-30 drops off from a maximum of 53 ft-lbs at 2,200 rpm to 46 ft-lbs at 3,200 rpms. Torque required at the prop to generate the power needed for 6.3 knots is about 53 ft-lbs, which isn't available at the rpms required. Consequently, I think you are likely looking at more like a 5 knot cruise speed with that engine and gear. Volvo also lists a 2.63:1 transmission as available with that engine. I think that would be a better choice since it makes more torque available at the prop. With the 2.63:1 gear and a 13" pitch prop you should be able to achieve maximum engine rpms at about 6.3 knots.

All of the above is predicated on the accuracy of Gerr's formulae in his prop book and will require ground truthing.


Todd,

Thank you for all the info. The engine is actually a new 2000 series model 2003. Volvo shows the turque curve to be a bit flatter for this motor than the 2030. 2003 is a larger displacement than 2030 (78 cu in vs 58.2 cu in).

Roughly 61 Nm @1500 rpm, 68 Nm @2300-2400 rpm & 63 Nm @3200. Hp at 2400 rpm is shown as 20.5 at the prop.

It is available from a private party so there is no option to change anything.

Thank you all for your time and information. It is appreciated.

Todd D
02-17-2010, 11:32 AM
Be wary of the prop power curve on a spec sheet. That curve is for a "typical" idealized prop/gear combination, not necessarily what you can expect in your own boat.

I have attached an example of a prop power curve calculated for your boat using Gerr's formulae. That curve also shows required engine torque and the torque curve you gave for the engine. I couldn't find the engine power curve for the MD2003, so I calculated the maximum available prop horse power as 28 - 3% loss in the gear minus 1 hp to run the alternator and on-engine pumps. This diagram suggests that you would be able to achieve 3,000 rpms with a 12" pitch prop. However, you will hit the wall on available torque at slightly lower engine rpms. An 11" pitch prop would let you get closer to 3200 rpms and shift the required torque curve to higher rpms.

http://todddunnmicroyachts.com/temp_files/volvo_power_diagram.jpg

willmarsh3
02-17-2010, 01:21 PM
The other thing to consider is parts availability and expense.

skaraborgcraft
02-17-2010, 03:39 PM
I had a hartly Tasman ferro boat,similar in size and weight,and installed a 10hp Bukh/westerbeke. It did throw a 14 inch prop,achieved hull speed and would push into a F8. What more do you need?
My friends used to run an old danish trawler,38 tons and 18 hp Saab....they didnt get in any trouble either.
Modern engines are all revs....get the biggest prop you can and get a gearbox to match. My 2 cents. Cheers

Stiletto
02-17-2010, 07:28 PM
I cant add anything to the technicalities already discussed, but perhaps another way of looking at the issue is to decide what would be your ideal choice of engine & characteristics thereof, and compare/contrast the Volvo on offer with that. Then decide if the required compromises are acceptable.

WannaB
02-18-2010, 11:45 AM
Gentleman,

Thank you all for your input.

I think this Volvo will work out. My concern was the 2.4 gear. But from info kindly provided here and a recent prop shop discussion it appears acceptable.

Dealer prices for new engines even discounted these days are still significantly higher. Despite being an older model, this is a new engine.

Compromises considered I don't feel it a bad option, particularly in this economy. A Kubota based engine would have been preferred I will admit.

I do appreciate all of your time and thoughts on this matter.

Now comes the joy of actually doing a swap!!

Thanks again.:)

skaraborgcraft
02-18-2010, 05:29 PM
Good luck with it,be sure to let us know the reality of the performance. The hardest part is always making the desicion. Cheers