Prop size calculator

I'm thinking of purchasing and re-powering a 1977 31' Searay Weekender. The original engines (2) are 255 HP gas, straight inboard Mercruisers. They currently don't run and I don't know what the transmission ratio is. The boat weighs about 9,500 lbs, and I'm told it cruised at approx. 22 MPH @ 3000 RPM.

I'm thinking of going to 320 HP EFI Mercruisers, also gas, and am wondering how I would figure out a couple of things. The engines are rated 320 @ 5000 rpm from Mercruiser and come with a 1 to 1 ratio tranmission.

Not that I would run it a 5000 RPM, but doesn't his seem like a bad combo for an inboard?

What's the maximum speed you would want to turn a prop in a configuration like this?

Is there a way to figure out what changes to the prop I would need to make with the new higher HP?

My goal is the best fuel economy at approx. 25-28 MPH cruise.

Thanks,

Dennis

 

Generally, the higher the prop rpm the smaller the prop diameter, and the less efficient the prop. You haven't told us what is the maximum prop diameter you can put on the boat (I assume that when you refer to the engines as "straight inboard Mercruisers" that you mean engines in the belly of the boat connected to the props via shafts, and not inboard/outboards with the engines mounted at the transom under the aft deck and directly connected to the exterior drive leg), which is a primary selection factor. It would also help if you could post the waterline beam and hull draft.

A quick check using my proprietory software indicates that the boat at 22 kts is running with about 2 x 150 hp. Extrapolating this out to twin 255's at full throttle, the boat ought to max out at around 26-27 kts. Swapping the 225's for 320's should put the max speed at around 31 knots, which may be on the upper limits of controlability of the hull. I'd do some searching about (try contacting SeaRay) to determine if this hull runs well at that speed.

I don't have any performance graphs of the Mercruiser 320 engine handy, but assuming that 250 hp x 2 is required for 27 knots, and assuming that the Merc 320 develops 250 hp at 80% throttle (4000 rpm) and the gear ratio is 1.01:1, then a 3-blade prop dimension of 14D x 12P would be in order and a 4-blade prop would be sized at 13D x 11P. This is optimized for cruising speed, so the engines may be overloaded if you try to max the throttles. To max out the new engines, you will need a prop of around 12D x 10P. This gives a prop tip speed of 262 feet per second (178 mph), which is a bit high. Talk to your engine/prop supplier before you commit to the engine/gearbox combo. You may want to look at a higher-ratio gearbox to allow a slower rpm, bigger diameter prop, if there is room under the hull to fit it.

 

MMD, if you don't mind, what does your program call for for a Chris Craft 26' flybridge, 10' beam, 9'4" on the waterline, 8500# dry, 10,500# loaded. She has twin Crusader 270s turning 17D x 16P wheels through 1.91:1 Velvet Drives. Lightly loaded in calm water she'll do 25 knots at 4400 RPM, and she has more throttle left (I've had them up to 5500 RPM) No room for any more diameter, prop shop suggested 20P, maybe with cup. What about 4 blade, would that be more efficient? Looking for range, mine's pretty poor, about 1 mpg. Also, when seas are rough, I run about 14-16 kt, turning 3200 rpm, with the boat dragging her tail. VERY inefficient, burning 16-18 GPH!!! Her most efficient speed is full out (mostly) 24 kt at about 4000-4200 RPM burning 20 gph (all by GPS and Flowscan), which is usually impossible to do offshore. By comparison with a Bertram 28 with the same engines, gears, (props unknown), that outweighs my boat by more than a ton. It'll cruise at 18 kt at 3200 rpm and top out at 24 kt at 4400. I should be able to blow that out of the water??!!!! I think 28-30 kt should be about right given the weight difference, and my Chris doesn't have near the deadrise of the Bert. My after body is nearly flat (she's shaped rather like a lobster hull, but with a shallower forefoot (don't know the deadrise). Anyway, Any advise would be appreciated. Thanks.

 

Craig, as you probably know, there are a ton of factors involved in getting a boat to run at the peak of it's performance capability, and props are just one of them. ChrisCraft is not known for it's stellar peformance, but more as a comfortable family boat. The shape of a hull has a lot to do with how fast it can go with a given power installation.

That being said, I think that your boat is a little low in the go department. I presume that you have trim tabs mounted on the transom; these can be a source for as much drag as they are benefit. Use them lightly or they'll rob you of a lot of speed. Also check your weight balance; if you tend to ride bow-high at cruising speed you may want to see if shifting some weight a bit forward might get the nose down and present a more efficient planing surface to the water. Experiment using weights such as plastic water tanks laid on the forecabin sole to see if there is an improvement. If so, consider moving permanent objects such as batteries, FW tanks, genset, etc., forward a bit. Also, the condition of your bottom is really important - it should be clean & smooth, and all edges should be quite sharp. Most importantly, weight is your greatest enemy. Put your boat an a diet and toss everything overboard that is not necessary. I bet that if you clean out lockers, get rid of unused items, and swap out a few old things for lighter, more modern stuff that you can shed a couple of hundred pounds off of your displaced weight.

Finally, the props. I agree with your prop guy - your pitch is a bit shallow for top end speed. They were probably sized as they are as a compromise to get out of the hole and up on plane quickly at the expense of top end and efficiency at speed. My numbers show that you should be looking at props in the 16D x 19P range to wring out top speed. Given that these numbers are 'guestimates', and you want to get as much blade area as possible in a given apeture, your prop guy's estimate of a 17D x 20P blade is probably on the money. Adding cup to the blades will help with the bottom end. Four blade props absorb more power, but at high rpms they run the risk of cavitating in the wash of the blade 'in front' as they rotate. Three blade props are probably best in your application.

Sadly, no matter what you do, that Bertram will probably always whip your butt. Ray Hunt is a genius at designing planing hulls.

 

Yup, I trim her down until the speed drops, then back them off. In calm water I can tell when they're in their "sweet spot" by the shape of the wake.

Unfortunately the weight is where it is on this one. A place for everything and everything in it's place. And no other place to go anyway!

Just got a fresh bottom job this week, and the performance numbers were from the ride back to the marina Wednesday.

Yup. She jumps outta the hole like a bass boat! I'm not pulling skiers, so that ain't important. So I guess I'll go with 20P 3 blade, and if they're too much, the prop shop can always pitch them back some, and forget about the cup.

I knew I should have bought that #@&! Bertram! Thanks a bunch.

 

Dennis,

send me some details of the boat:
- Length over all,
- Length on water line,
- breadth max.,
- breadth on w.l.
- draft.
- max. allowable diameter of the propeller.
You said that the weight 9500 lbs.

If you have one photo (out of the water), would be nice.

rgds

Davor

 

I'll send the info later this afternoon. Although the boat is out of the water, I wouldn't be able to get a picture until Wednesday.
Dennis

 

Resistance / propellers

Dennis,

I will use my resistance / propeller calculation program and will send you solution.

If you are interested in these calculations look at my
website www.sea-power.net , but it is for engineers.

Davor

 

OK, here's what I have on the Searay;
Gunwale length 30' 11"
Centerline length 29' 7"
Waterline length 27'
Transom width at gunwale 11'
Transom width at chine 9' 7"
Deadrise at the transom 18 degrees
Draft 29"
Weight 9,000 lbs.
Propellers (2) - I could only take a measurement with a tape under the boat and from the center of the shaft to the tip of one of the blades it measures approx 8.5". I could not determine pitch. The prop cleared the bottom of the hull by only 1". There are no pockets.

Again, my goal is cruise economy.
Thanks for all of the input.
Dennis

 

Results for Dennis

Hi Dennis,

Here are my calcultions based on data you sent me.
Find attached:
1. Resistance calculation (Radojcic method)
2. Three propeller calculations (for 2200, 2500 and 2800 RPM)

According to resistance calc. Your opt fuel econlomy will be between 28 and 30 knots.
Above 30 knots power is going up. With these two engines you will be able to go to max. speed about 32 knots. 10 % Sea margine is included in calculation.

Note that in resistance calculation the power on the graph is delivered power and in your case it is nearly breake power.

Eta_p applied was 0.55 ant it matches very well efficiency of the propellers.

So conclusion is as follows:

1. RPM opt is between 2300 and 2600 RPM , and transmission ratio will be between 1.9 and 2.1. Look at attached graphs, at 2800 RPM propellers are in cavitating area.
Look for gear box closest to above figures, and select propeller (ftom table below).
2. Calculated propellers are :
2200 RPM 2500 RPM 2800 RPM
------------------------------------------------------------------
Ae/Ao > 0.85 > 0.85 > 0.85
Diameter 17’’ 17’’ 17’’
Pitch 19’’ 17’’ 15’’
Pitch / diameter 1.13 0.99 0.875
Numer of blades 3 3 3

I calculated all three propellers for max diameter you measured, as bigger diameter results with better efficiency.


Awaitting your comments

Davor

 

Propellers

Hi Dennis,

have you sean my calculations? Any comment?

rgds

Davor

 

Davor,
Thanks for the calc's, although I'm not quite sure I understand them all. I'll study them some more and see if the light goes on in my head.

You mention that the optimum RPM is 2300-2600 RPM and the prop is close to cavitating at 2800. You also mention that via the resistance meathod, the most economical cruise is 28-30 knots? What are the engine RPM"s at 28-30 knots. It seems that they would be quite high.

Is this too much engine for the boat? Should I just get less HP and be happy with the lower cruise speed? How do people who put diesels in these boats get bigger props under the boat? I can't imagine that they could get away with the 17" diameter wheels that boat has now and the hull won't allow anything larger. Would it help if I used jackshafts to duo-prop outdrives instead?

Thanks,
Dennis

 

Dennis,

The diameter of the propellers is o.k. To absorbe more power for the same diameter you have corresponding pitch and area of blades.

I calculated 3 propellers for 3 different RPMs. The aim was that you find adequate transmission ( between 1:1.9 and 1:2.1) and than by interpolation (from my table) calculate pitch of your propeller.

You can see from the table that in all cases Diameter is 17", 3 blades and Expanded area ratio (Ae/Ao) is greater then 0.85. So you can look for the pitch only.

Let me know your transmission ratio.

rgds

Davor

 

http://www.mercurymarine.com/prop_selector/