Prop size, again....

Discussion in 'Inboards' started by firepiper, Oct 11, 2008.

  1. firepiper
    Joined: Aug 2006
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    Location: Plymouth, Ma

    firepiper Junior Member

    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
     
  2. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    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?
     
  3. firepiper
    Joined: Aug 2006
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    firepiper Junior Member

    Yes, doesn't seem to add up...

    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.
     
  4. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    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.
     

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  5. firepiper
    Joined: Aug 2006
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    firepiper Junior Member

    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
     
  6. Guest625101138

    Guest625101138 Previous Member

    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
     

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  7. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    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.
     
  8. firepiper
    Joined: Aug 2006
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    firepiper Junior Member

    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
     

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  9. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    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.
     
  10. firepiper
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    firepiper Junior Member

    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
     
  11. terhohalme
    Joined: Jun 2003
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    terhohalme BEng Boat Technology

    Propeller

    Propcalc (Castle marine) says:

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

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  12. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    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?
     
  13. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    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?
     
  14. firepiper
    Joined: Aug 2006
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    firepiper Junior Member

    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
     

  15. firepiper
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    Location: Plymouth, Ma

    firepiper Junior Member

    very similar waterlines, sorry
     
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