Prop size, again....

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

 

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.

 

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

 

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

 

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

 

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 *** 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

 

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.

 

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

 

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

 

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.

 

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.

 

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/listi...rency=EUR&access=Public&listing_id=20424&url=
http://it.yachtworld.com/core/listi...rrency=EUR&access=Public&listing_id=1821&url=
http://it.yachtworld.com/core/listi...rency=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.

 

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

 

Gear Ratio"s

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?