Prop size, again....

[firepiper]

1987 Luhrs Alura 35, down east style, 3/4 keel, twin f.i. Crusader 454, model 350. 1.9:1 Vevets, right now turning 18x18 3 blades nib.

4300 rpm 26mph/22.5 kt MAX

I cruise at 3000 around 17kt in flat water. I would like to cruise more like 26-2800 rpm @ around 18-20 kt.

I repowered this boat. It had 350 c.i. small blocks with these same props and trans.

Any help would be great, thanks, Mark

firepiper.ffd@charter.net

[daiquiri]

It just doesn't add up to me. :confused:
With 700 HP installed on 35 ft hull your max speed is only 22 kts? Do you know what is the boat's weight? Is it some kind of semi-planing hull?

[firepiper]

Boat weighs 15000. Clean bottom. Fellow Alura owner says his 8.1's max out at 30 kt. Same boat, touch more hp.
I have trim tabs and can trim it out nice. I charter the boat for fish, and have a 22 mile trip across the bay, I need more effieciency. Once I getthere ,I drop an engine to troll. I just know it can be better.

[daiquiri]

For some reason, your boat has a different resistance curve than the other guy's Alura. Maybe it has a different weight, CG position, or whatever. You should investigate into this if you want to approach his 30 kts. You may try to talk to him and see where the difference is. Ask him what props does he mounts and what is his gear ratio.

By the way (and sorry for the stupid question), are you sure you're getting 26 mph and not 26 kts? :!:

In the meanwhile, I've made few calculations with your data (and having a confidence that you have already checked the above). ;)

Let's start with your actual cruise configuration. I have modeled your boat's caracteristics according to this one, because I have a suspect that at full throttle your prop cavitates, so the cruise condition is more indicative.
You can see it in the pic n.1 - 17 kts, 80% throttle, 3000 rpm, so it's ok.
Now let's give it 100% throttle - pic no.2 .
It appears that you should not be able to go over 3600 rpm with the prop running properly. Since you say that you arrive to 4300 rpm, it means that the difference between 4300 and 3600 rpm is really a power waisted to rev the stalling (or cavitating) prop, rather than to move the boat forward.

One of the solutions could be to try installing a 3-bladed 17"x16" prop. This one should allow you to run the engine at full RPMs at 26 kts, and blades will work properly (pic no.3).
At 75% throttle you will then be able to cruise at 18 kts and at about 3500 RPM (pic. no.4).

You can't have 2600-2800 rpms at 17 kts without changing the gear ratio, imho.

But let me repeat one more time: check the mph/kts data and try to talk to the other Alura owners to see what's so different in your boat's configuration.

[firepiper]

Went out on the boat today. Here's the numbers:
3000 rpm-17 kt (20mph)
4300 rpm-23 kt (27 mph)
Boat is 15000 lbs. Twin Crusader MPI 454 (350 hp)
Velvet drive 1.91:1 trans

I spoke with the other Alura owner, He has 8.1 Crusaders w/ 1.5:1 trans
17x17 3 blade wheels
He says 30kt @ wot (4400). Everything else on boat is same.

Does this help? I'm looking to troll one one engine, around 3.0kt (3.5mph) idle (approx), and cruise, (two engines) at the most efficient speed, hopefully around 22-24 kt. Thank you so much for your help, Mark

[Rick Willoughby]

Boat weighs 15000. Clean bottom. Fellow Alura owner says his 8.1's max out at 30 kt. Same boat, touch more hp.
I have trim tabs and can trim it out nice. I charter the boat for fish, and have a 22 mile trip across the bay, I need more effieciency. Once I getthere ,I drop an engine to troll. I just know it can be better.

The prop numbers sort of align with the motor rating for the power absorbed but the boat drag for the engine/prop conditions given is much higher than expected. Something like twice what you would expect.

The attached screen image is from a drag prediction model for a planing hull. It shows the web site if you want to check it out. It indicates that a hull like yours at 15000lb (6800kg) should require 220HP to do 30kts. So for twin engine only 110HP per engine.

You are apparently using much more than this so the difference needs to be explained. The 18X18 props will be achieving around 70% efficiency under the loading conditions you provide but they would be better if the drag was more in line with the prediction. An 18X18 prop at 2260rpm will absorb 285HP at 23kts. The thrust (from EACH prop) will be 13000kN (2918lbf) compared with TOTAL drag prediction of 1980lbf at 25kts so well above twice what should be required.

The things that can consume extra power are:
1. High windage. Do you have an enclosed flybridge with lots of things hanging off it.

2. You say the hull is clean - how clean. Any small barnacles; any slime?

3. Props fouled - again any barnacles.

4. The keel sounds odd. Does it end abruptly or is it nicely faired.

5. Forcing the bow down with trim tabs to get a better ride could reduce speed. Have you played with trim to find the best economy position.

6. The boat is heavier than you have provided. Should be obvious by sitting deeper in the water.

7. A combination of any/all of the above.

The props could be aerating but this will not result in great loss of efficiency but will go slower. You should be able to check as the engines will be at rev limit not at torque limit. Also check the fuel rate at top speed to verify if you are getting full engine power.

Rick W

[daiquiri]

Your engines also might not deliver the rated power, so you need to check the following:

1) Check the boat's air intakes, to see if they are choked in some manner (or by something). Your engine requires about 70000 cu.ft/h of air.
2) Check the engines' air filters, for the same reason.
3) Check the fuel piping, from the tank to the engine. It might be choked or not sealed properly, so the engine may suck an air-fuel mixture.
4) Check the spark plugs and their electrical wiring. It may happen that an engine runs with one or more cylinders inoperative because the spark plugs are not functioning or their electrical wiring is worn out.
5) Check the fuel pump.

If you don't know how to do it or what to look for, ask an engine mechanic to do this for you.

[firepiper]

Thanks. I've checked the electric fuel pumps, lines, and air intakes. The engines run mint, plugs are clean and the spark arrestors also. I've also run it up with the engine covers open, no difference.
I've attached a pic of the boat/hull/keel

[daiquiri]

What is the boat's trim at max/cruising speeds? I see the waterline in the photo more pitched up than what seems to be the design waterline in the (btw very small) pic to the right.

[firepiper]

I'm not quite sure what you mean. I do not have a trim 'guage'. I give it bow down until my speed maxes out at a given rpm.....does that make sense?
The small pic was to show the keel. Marl

[terhohalme]

Propcalc (Castle marine) says:

30 mph with 19 1/2 x 16 3/4 propeller

[daiquiri]

Propcalc (Castle marine) says:
30 mph with 19 1/2 x 16 3/4 propeller

What is the number of blades and E.A.R. of the props you have used in the calculation? Also, what manufacturer has them in the catalog?

[daiquiri]

I'm not quite sure what you mean. I do not have a trim 'guage'. I give it bow down until my speed maxes out at a given rpm.....does that make sense?

I'm basicaly grasping at straws trying to understand why does your boat have such a high resistance compared to the other similar boat.
So I notice that in the first pic your waterline is inclined, looks like C.G. is pretty aft. Did you modify the hull, engine room or plants in some significant way?
Second thing - did this boat, as far as you know, ever perform better than this?

[firepiper]

I replaced the original small blocks with big block engines.
All the 35 Aluras I have seen, have that 'squat' toward the stern. Any similar waterlines. Mark

[firepiper]

very similar waterlines, sorry

[daiquiri]

I'm about to give up.

You need to have an experienced person who will step on your boat, make a thorough visual inspection, possibly measure her weight and lcg position and perform a range of test runs to fully undestand where is the problem.

I (or anyone else here) could suggest you a new prop to install but it would be a wrong thing to do, because at this point it would be based on input data of unknown validity. First you need a reliable assessment of the situation.

That's the best suggestion I can give you at this point, while sitting at the other part of the globe.

Cheers mate, and let me know what did you discover. I got really curious about this.

[firepiper]

Well thank you. What kind of tests are you thinking? I'm pulling the boat next week, and wanted to order props over the winter. So, i'll have to act fast.

[terhohalme]

Propcalc uses "3 blade std. propeller" what ever it means. More information on their site:

http://www.castlemarine.co.uk/

[Rick Willoughby]

1987 Luhrs Alura 35, down east style, 3/4 keel, twin f.i. Crusader 454, model 350. 1.9:1 Vevets, right now turning 18x18 3 blades nib.

4300 rpm 26mph/22.5 kt MAX

I cruise at 3000 around 17kt in flat water. I would like to cruise more like 26-2800 rpm @ around 18-20 kt.

I repowered this boat. It had 350 c.i. small blocks with these same props and trans.

Any help would be great, thanks, Mark

firepiper.ffd@charter.net

Lets put the high drag aside and assume it is the hull/keel shape. The 18X18 props will consume about 85HP each at 1578rpm doing 17kts. THe efficiency will be about 73%.

If you can fit 20 X 18 then prop rpm will reduce to 1510rpm and prop efficiency will lift to 76%. A 20 X 19 prop will drop rpm to 1450 and the efficiency stays at 76%.

If you have room to fit larger diameter then this will improve prop efficiency by reducing slip so the rpm drops a little as well. Obviously increasing the pitch will reduce engine speed. An 18X20 prop will give also give 17kts at 1450rpm but the prop efficiency remains at 73%.

The motors will certainly have the torque to run the higher pitch around the 17kt speed and you should get better engine efficiency. The downside is that you may reduce top speed because the engines will not rev out.

If the slip has a weight scale it would pay to check the weight.

I would like to see a photo of the back of the keel and the bottom of the hull around the props.

One test to do now is to do a steady run at your desired speed, say 18kts, and get an accurate fuel burn. At least this will provide a baseline to compare any changes.

I would expect that working the engine harder at lower rpm will improve motor efficiency.

Rick W

[firepiper]

I'll get the pics for you in the a.m. (i'm at work,pics are home). I can tell you that the keel is about 5 inches wide and is square at the edges. The bottom is smooth and without obstructions.
My only fear of increasing diameter is I need to be able to troll on one engine around 3-3.5 mph. Mark

[daiquiri]

Rick, we do agree on the figures about his cruise speed. It will be around 17-18 kts, be it one prop or another (reasonably chosen, of course).
The problem, as I see it, is this boat's top speed. The other guy's boat runs at 30 kts while this one does 22 and will make it up to max. 24-25 kts with new props.

So I believe that, while it's ok to advice firepiper about the best prop for his current boat configuration, there remains the fact that this boat is for some reason hydronamicaly inefficient. In a sense that it has a high resistance compared to the other similar boat - and that will spoil the overall propulsive efficiency whatever prop you choose to install.
That issue needs to be resolved first - if possible.
If, after all have been done to improve the boats' characteristics, firepiper sees that it is related to some non-modifiable feature like engines weight and position, interior structures, ifurnitures or whatever - than he accepts the things as they are and choose the best prop for the boat he has.
IMHO, as always.

Btw, your prop efficiency estimates are still so optimistic... I mean, 73-76%...

[Rick Willoughby]

I'll get the pics for you in the a.m. (i'm at work,pics are home). I can tell you that the keel is about 5 inches wide and is square at the edges. The bottom is smooth and without obstructions.
My only fear of increasing diameter is I need to be able to troll on one engine around 3-3.5 mph. Mark

The bigger diameter improves grip. I cannot give exact numbers but the relative figures something like 800rpm on engine give 3kts with 18X18 then the bigger prop will only need 660rpm to do 3kts with 20X18. So you are right to be concerned.

So something else to check is the current rpm at slowest required trolling speed. You could need something like 20% less with the bigger diameter prop.

Rick

[daiquiri]

By the way - firepiper, are you sure your friend is not giving you mph numbers instead of kts??? That's very important and is a common mistake. 30 mph would be only 26 kts - in that case you just put the right prop and your boat is ok! :!:

[firepiper]

I'm starting to wonder. But, even then, i'm not quite at 26 kt (23kt), and would still like to drop some cruise rpm's. (<3000) He's not a friend, just a guy that has a similar setup (8.1's w/1.5's) on the same boat. Now i'm more confused!!

[Rick Willoughby]

Rick, we do agree on the figures about his cruise speed. It will be around 17-18 kts, be it one prop or another (reasonably chosen, of course).
The problem, as I see it, is this boat's top speed. The other guy's boat runs at 30 kts while this one does 22 and will make it up to max. 24-25 kts with new props.

So I believe that, while it's ok to advice firepiper about the best prop for his current boat configuration, there remains the fact that this boat is for some reason hydronamicaly inefficient. In a sense that it has a high resistance compared to the other similar boat - and that will spoil the overall propulsive efficiency whatever prop you choose to install.
That issue needs to be resolved first - if possible.
If, after all have been done to improve the boats' characteristics, firepiper sees that it is related to some non-modifiable feature like engines weight and position, interior structures, ifurnitures or whatever - than he accepts the things as they are and choose the best prop for the boat he has.
IMHO, as always.

Btw, your prop efficiency estimates are still so optimistic... I mean, 73-76%...

I get the impression that the hull is not designed for high speed. From what I can gather it has a deep entry and large keel that is chopped off so a lot of extra drag compared with a more usual planing hull. The cabin is typical and no flybridge so no more windage than usual.

The prop efficiencies given are always the best case. Irrespective the relativity is the important consideration. Going up in diameter will improve efficiency and drop engine revs for any given speed.

Rick W

[Rick Willoughby]

I'm starting to wonder. But, even then, i'm not quite at 26 kt (23kt), and would still like to drop some cruise rpm's. (<3000) He's not a friend, just a guy that has a similar setup (8.1's w/1.5's) on the same boat. Now i'm more confused!!

Any idea on the props being used on this boat?

Rick W.

[daiquiri]

I get the impression that the hull is not designed for high speed. From what I can gather it has a deep entry and large keel that is chopped off so a lot of extra drag compared with a more usual planing hull. The cabin is typical and no flybridge so no more windage than usual.
The prop efficiencies given are always the best case. Irrespective the relativity is the important consideration. Going up in diameter will improve efficiency and drop engine revs for any given speed.
Rick W

I believe that, since we are dealing the ordinary-real-world boats, it would be more correct to give efficiencies as close to reality as possible. It is useless to tell the person that his prop will have 74% efficiency, knowing that you still have to detract a lots of things from that number.
I recognize that even 63-64% which comes out of my spreadsheet is optimistic for the comercial props of this size, but at least is trying to get closer to the reality.

Don't misunderstand me, Rick - I'm in no way questioning the corectness of your methods (which are proven, as we know), but I'm trying to put myself in the skin of persons like firepiper who are not prop experts yet need to have some real numbers that apply as close as possible to his boat.

Now, about this case. I think pretty much the same thing - that this boat is not designed for high speeds. and I believe that the other guy is really planing at 26 kts (30 mph). And that is the speed that can be approached.

This is my final result, I've put the corrected boat's weight in the spreadsheet:
3-bladed, 0.61 E.A.R. Michigan Wheel type prop, 18"x15" should give 25 kts top speed at 4550 engine rpm.
Cruise speed at 75% throttle will be 19 kts at 3700 rpm.

Both max and cruise engine rpms would be correct for this engine, for the reasons I have explained in this thread (that guy had the same engine):
http://www.boatdesign.net/forums/showpost.php?p=231072&postcount=15

And the last phrase in that post also remains valid. :p

[Rick Willoughby]

I believe that, since we are dealing the ordinary-real-world boats, it would be more correct to give efficiencies as close to reality as possible. It is useless to tell the person that his prop will have 74% efficiency, knowing that you still have to detract a lots of things from that number.
I recognize that even 63-64% which comes out of my spreadsheet is optimistic for the comercial props of this size, but at least is trying to get closer to the reality.

Don't misunderstand me, Rick - I'm in no way questioning the corectness of your methods (which are proven, as we know), but I'm trying to put myself in the skin of persons like firepiper who are not prop experts yet need to have some real numbers that apply as close as possible to his boat.

Now, about this case. I think pretty much the same thing - that this boat is not designed for high speeds. and I believe that the other guy is really planing at 26 kts (30 mph). And that is the speed that can be approached.

This is my final result, I've put the corrected boat's weight in the spreadsheet:
3-bladed, 0.61 E.A.R. Michigan Wheel type prop, 18"x15" should give 25 kts top speed at 4550 engine rpm.
Cruise speed at 75% throttle will be 19 kts at 3700 rpm.

Both max and cruise engine rpms would be correct for this engine, for the reasons I have explained in this thread (that guy had the same engine):
http://www.boatdesign.net/forums/showpost.php?p=231072&postcount=15

And the last phrase in that post also remains valid. :p

Do not disagree with the result but it does not answer the original question:

"I would like to cruise more like 26-2800 rpm @ around 18-20 kt. "

I am suggesting a 20X18 would be a good choice to achieve this.

Are you recommending the only option is to fit an 18X15 and run the engine at 3700rpm??

Rick W.

[daiquiri]

Do not disagree with the result but it does not answer the original question:
"I would like to cruise more like 26-2800 rpm @ around 18-20 kt. "
I am suggesting a 20X18 would be a good choice to achieve this.
Are you recommending the only option is to fit an 18X15 and run the engine at 3700rpm??
Rick W.

I've had to work till very late last night to respect a deadline of a project I'm doing and it was very late when I wrote the last post.
So I'm here right now principaly to give you my public apologies for having criticized your results in public rather than discussing those questions in private with you. It is not my style and just didn't think about how would I feel if someone did it to me. Your competence is proven, I respect your work and I believe that Firepiper can trust your advices.
So, once again, sorry for my lack of style. :o

I agree with you that a 20x18 prop will give about 18 kts@2900 rpm, with some 90% throttle, I guess. Depends principaly on how you model the boat's resistance.

For my calculus, I have taken into a consideration the engine data too. JSH gave us the curves for Crusader 454 engine here:
http://www.boatdesign.net/forums/showpost.php?p=230858&postcount=13
For the reasons I have quoted in my previous post, the correct RPM range for this engine should be between 3500 and 4600.
So, although it is true that Firepiper wanted to run at 2900 rpm, I think he should be adviced for higher regimes, because at 2900 this engine's torque is in the instable regime, i.e. it needs to be compensated manualy for the increase/decrease in resistance (due to wave activity or wind, for example).

Once I've adviced him on that, I know I did my best and it becomes a matter of Firepiper's choice. :)

Now let me explain how did I choose the prop.
A 3x18"x15" E.A.R. 0.61 propeller allows the engine to work in it's stable regime of torque - at 3700 RPM at cruise and 4500 RPM max speed.
Diameter 18" is identical to the prop already installed, so I have taken it as upper limit for the diameter because it already fits the available space under the hull and probably allows for correct clearances between the prop and hull.
I have choosen the pitch 15" considering the above reasoning about the RPMs.
The number of blades and E.A.R. are chosen taking care of cavitation issues.
Finaly, the prop needs to be readily available in boat shops, so I have considered the commercial series only.

So, I absolutely don't say it is the only prop possible. and it is not the most efficient one for the cruise. This one simply appears to be the which satisfies the above design considerations.

That's all. I repeat, from the purely hydrodynamic point of view regarding the props characteristics I agree completely with your results - it's just that you can't consider this separated from the engine characteristics, imho.

Best regards

[Rick Willoughby]

.........
I agree with you that a 20x18 prop will give about 18 kts@2900 rpm, with some 90% throttle, I guess. Depends principaly on how you model the boat's resistance.

........

For the load calculation at 17kts I determined what power the 18X18 prop would absorb at 1510rpm. This was 85HP at the prop so say 100HP allowing for auxiliaries and shaft/box losses. This is well within the engines power capability. Will deliver 230HP at 3000rpm. Fuel burn per engine from engine data is estimated at 43lb/hr.

Going to the 20X18 prop the power required to do 17kts drops to 82HP or say 97HP allowing for auxiliaries and other losses. Fuel rate drops to 41lb/hr.

The engine data only gives full throttle power and best efficiency is around 3600rpm. Would be reluctant to assume best efficiency at lower throttle is at the same rpm.

The 18X15 prop gives 17kts at 3400rpm and prop power is the same as the 18X18. There may be a slight advantage as the engine could be in a more efficient range but you would want to see fuel rate at lower throttle settings to confirm this.

The 43lb/hr on each engine should be equivalent 12.4 US gallons per hour for both.

Rick W.

[firepiper]

Hello again. I just ulled the boat to see actually what was on there for size, and also the condition of the bottom,(cleanliness). I was very happy to see the bottom was super clean, no slime, weeds or other growth. I guess that expensive bottom paint is really worth it. I found there were some barnicle growth on my trim tabs, not many.
As for th props, I was wrong, they are 18x16 3 blade, not 18x18 as previously stated. What does this change....and where do I go from here? Thank you guys so much for your help, Mark

[daiquiri]

I was wrong, they are 18x16 3 blade, not 18x18 as previously stated. What does this change....and where do I go from here?

If the other numbers regarding the actual top rpms and speed are still valid, then I think that you might gain maybe a knot at full throttle by installing a 18x15 prop. It will allow the engine to run at max rpms thus giving you the max power. The price for that is some 5% loss for NMPG value at cruise speed.
You decide if it is worth it.

[Rick Willoughby]

An 18 X 16 prop at 2263rpm and 26mph will absorb around 200HP. Considerably less than the 18 X 18 for the original numbers you supplied.

It means the estimated drag is somewhat less than the previous estimate. The thrust estimate is now 10.5kN per prop. I also looked at the numbers for the other boat at 30kts with 17 X17 props at 2933rpm. It comes out at 14kN thrust. These number equate to 370HP per engine. Looks too high for engines rated at 350HP.

While these drag figures are higher than you would expect for 15000lb boat they are reasonably consistent for the speed difference.

The point now is that your motors do not seem to be developing anything like the rated horsepower. The difference between 22.5kts and 30kts is huge in terms of the power required to achieve it.

There is a bit more consistency in the drag data between the two boats but your data looks bad for the motors and the data for the other boat looks too good to be true if the motors are only 350HP.

Factors that can make a difference besides weight and clean bottom are:
1. Weight distribution. Heavy by the stern will mean you have a real drag hump and somewhere around 25kts will be the lowest drag. Getting weight forward will reduce drag overall and make it easier to get on the plane.

2. Prop shaft inclination. Check if they are the same. I do not like inclined shafts but it is a fact of life. The other boat with smaller diameter props may have shallower angle.

3. I am assuming all the data you supply is correct. The best way to get speed is on a calm day where there is no current using a GPS. If you cannot guarantee no current then you need to do runs in two directions roughly at the same time. Prop rpm is the important thing. So check the gearbox ratio and make sure the tacho is reliable. A calibrated tacho on the prop shaft would be best.

The fact remains that 30kts and 22.5kts are hugely different for nominally the same boat. It should not be difficult to arrive at the difference. From the basic data you have provided it would seem the engines in the other boat can deliver 370HP compared with your 200HP. His props are not much different to yours on the face of it yet he can spin his much faster.

One other aspect of interest would be the EAR of the props. If his have a lower EAR then that would improve efficiency and reduce my power estimate somewhat.

Rick W

[firepiper]

The two hulls are identical. Same year, model, make, length. The engine and trans are the difference. His engines are 8.1l and his trans are 1.5:1. I do not know what those 8.1's put out for h.p. My speed calcs were done as you said, gps in each direction. The one variable would be the tachs, though, they are brand new. Thanks, Mark

[Rick Willoughby]

There are two 8.1l engines shown on the crusader site:
http://www.crusaderengines.com/Products/8_1.html
The 385HP fits reasonably well with my estimate of 370HP at 30kts. If it was the 425HP model then it starts to get closer to the power difference expected for 22.5 to 30kts but it would mean my power calc from the 17X17 prop is somewhat low.

There is no current 454 model. Can you find something that has the engine specs. I am assuming from the designation it is 350HP but the GM 454 series range down to 310HP.

The difference between 22.5kts and 30kts being achieved with 35HP difference would require a droop in the drag curve. This could come about by significant stern down trim. If this was the case you should find the boat speeding up quite a lot in a following sea as it gets fully on the plane and levels out.

Rick W

[firepiper]

Rick, my 454's are supposedly (when new) rated 350 hp. The man with the new 8.1's are 385 hp. I feel that my engines are running excellent, I tore them down last fall prior to installing. Checking things like bearing clearances, and resealing. The compression tests were right on the money, averaging 145psi. These engines were retrofitted with Cutler (now Holley) multi port fuel injection systems, computer controlled. They purr!! I really don't think it's the engines. I have researched several pictures of other Alura 35's. They all seem to sit heavy in ass end, as judged by the factory water line/ boot stripe.
I realize that upsizing from small block chevy's, to what I have now would add some weight, I also added a swim platform, but, cannot believe the weight would be enough to cause an issue. I was discussing the findings you and others have come to with my local prop shop guy. Who, buy the way, doesn't like to discuss his methods.....he wants me to go with a 19x19. This is not my plan, and I am looking for another reputable prop shop. It's not that i'm against his idea, it's that he won't share his math or formula in which he is coming up with this. He acts like it's a trade secret or something. I just want an effiecient boat, that suits my needs. Thanks, Mark

[Rick Willoughby]

Mark
This thread has some interesting comparisons with your dilemma:
http://www.boatdesign.net/forums/projects-proposals/hull-modifications-23205.html

The boat is considerably heavier and shorter. It is now getting 19kts with 500HP. The big diameter props lift efficiency over what you have but then the boat is 25% heavier. You can see why something looks wrong with your data.

Going up in prop diameter to 19" will help efficiency. Going for the larger pitch will bring the revs down but may actually prevent the motors revving out so top speed could be lower. What is the EAR of the recommended props?

The one detail you have not confirmed with the other boat is the angle of the shafts. Are they the same as yours?

Rick W.

[firepiper]

Yes, the angle is the same. He told me they made no mods to the shaft angle. Mark

[daiquiri]

Rick,
You can find the 454 engine curve in this discussion:
http://www.boatdesign.net/forums/inboards/prop-size-24423.html
Go to the post #13 and you'll see the pdf file.

[firepiper]

I almost forgot...He also told me the factory weight is way off. His boatyard weighed it at 19600, with the new engines. Mark

[firepiper]

Daiquiri, What do you mean? I don't understand...

What It appears that you should not be able to go over 3600 rpm with the prop running properly. Since you say that you arrive to 4300 rpm, it means that the difference between 4300 and 3600 rpm is really a power waisted to rev the stalling (or cavitating) prop, rather than to move the boat forward.

[Rick Willoughby]

Rick,
You can find the 454 engine curve in this discussion:
http://www.boatdesign.net/forums/inboards/prop-size-24423.html
Go to the post #13 and you'll see the pdf file.

I have seen that plot and wonder what it applies to. The original post on the thread stated as follows:
Power is a 7.4l crusader ch350 (through velvet drive 72c 1:1) develops 300 hp @ 4500
I would like to run the motor at the 3500rpm range developing 200hp+/- with lots of torque.

So is this right or do I believe a very dodgy looking curve.


I could not find manufacturer's data for a current model Crusader 454.

daiquiri - We now know the props are 18X16 so work out what power they should absorb at various speeds. I cannot get them taking 350HP at 22.5kts at the nominated rpm.

Also the boat has got heavier and is now closer to the weight of Blizzard that MKP has provided some great data on. It will do 19kts with 500HP and has a long, relatively deep keel. The one advantage is the nice big props:
http://www.boatdesign.net/forums/projects-proposals/hull-modifications-23205-4.html

Rick W.

[daiquiri]

Daiquiri, What do you mean? I don't understand...

Nothing, it is no more valid. It was the result based on the old data.

I have ran once again the simulation with input data relative to max speed you have achieved during the test run and it corresponds well with the results of the calculation.

Your boat is ok (for what it was designed for), the max speed you could expect with a boat weighing 13000 lbs would be around 26 kts. See these other Aluras:

http://it.yachtworld.com/core/listing/boatFullDetails.jsp?boat_id=1984226&ybw=&units=Meters&currency=EUR&access=Public&listing_id=20424&url=
http://it.yachtworld.com/core/listing/boatFullDetails.jsp?boat_id=1891737&ybw=&units=Meters&currency=EUR&access=Public&listing_id=1821&url=
http://it.yachtworld.com/core/listing/boatFullDetails.jsp?boat_id=1654161&ybw=&units=Meters&currency=EUR&access=Public&listing_id=71493&url=

Your boat weighs more (15000 lbs), has a cg somewhat more aft than the lighter-weight Alluras and that could be the reason, imho, why it doesn't arrive to their speed.
The guy who told you he arrives at 30 kts is probably mistakingly using mph instead of kts. 30 mph equals 26 kts.

You can try to reduce the prop pitch by 1" to let your engine arrive at full rpms (and full power). That could make you gain maybe a knot, but I think that's all you can have with the boat in this configuration. I would like to hear if Rick has any other news.

[Rick Willoughby]

Mark
The attached shows the sensitivity to aft trim. I have the lcg way aft. The sort of drag curve is really noticeable by the size of the wave as the boat lifts onto plane.

If you want to try your own data go to this link and try a few values that might be more accurate than my guesses:
http://illustrations.marin.ntnu.no/hydrodynamics/resistance/planing/index.html

Going up in weight gets the drag up somewhat but it is still well under what is required to do 22.5kts in your case if you are delivering all 700 horses to the props.

Any number of people should be able to give you the answer to the problem posed here:
What HP will an 18X16 prop absorb at 22.5kts spinning at 4300/1.9rpm?

You might get a range but I would be surprised if anyone can get as high as 350HP. By my calculation that prop would need to be doing 2600rpm (say 4900 at motor) to pull 350HP.

Look at the claimed speeds of the boats in the previous links and the installed HP. I also wonder if the weights are correct because they are all the same and much lighter than you have.

Rick W

[THUNDERCAT]

RACING INFALTABLES.
4.1 metre 320 kgs fully laden, catamarran hull,we have achieved 104km/h with a modified Yamaha 50hp at 6450 rpm with a 18 pitch full clever,using a 1:85 standard Yamaha box. I have changed the box with a 60hp twins box ratio being 1:71 Theoritically this would give me another 9km/h per hour at 6500 rpm.The motor cant get past 4200rpm???. The modified 50hp is throated,compression ratio is at 11(from 8.00) cold crank is 13 bar.uses 60 horse carbs.On the scale it shows that as the rpm increases the torque falls off the chart by 10ftpounds??.any ideas as to how to fix this or do we have to get closer to a 60hp torque caperbilities?

[adventuris]

firepiper i ran some prop numbers ,if your engines are running correctly then u need 19 x 19 3 blade just like your prop guy told u .i ran you speed rpm numbers back to get a horesepower you are putting out , i come up with u are only producing about 225 hp .i am thinking you have an engine issue do oyu have the correctly sized holley fuel injection setup on those engines ,sounds as they are not getting the fuel they need .with your props know u shouldbe turning over 5000 rpms.

[THUNDERCAT]

thanx ,but you are off the page.its a 50hp yamaha.cant use anything else.just changed the gearbox ratio"s.