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Speed of my boat at full speed

M

mnakkach

New member
Hello,

I am new this forum, I lately purchased a used boat, it is 24ft fishing boat, 2400LB without engines, it has 2 Yamaha 115hp Enduro twin engines, at full speed 4500rpm it was able to reach only 24miles per hour (40kmh).

Is this normal? is there a kind of calculator to know the approx. speed based on engine HP and boat size?

Thank you
 
Sounds too slow. could be a lot of things though, so more info required. Does the boat live in the water or on a trailer?
 
Thank you everyone for your help.

The boat is not much famous in your place, it is Gulf Craft fisherman 2400, weight is 2600lb, 25feet, v hull.

Yes,it lives on salt water, I will get some more information about it after I receive it during this week. I had a test drive and this was my findings, maybe the hull has some algae. will inspect it more soon and update you.
 
Gulf Craft from Dubai? I used to sell them a few years ago. Was out in an old clients GC WA31 a few days back. If it is a Dubai GC it is likely a balsa core hull, so shouldn't be overly heavy unless water logged from some damage etc. From memory the GC WA31 will do over 30knts with a pair of 115's, so your speed is well down as are your WOT rpms. Most likely cause is a dirty hull if it lives in salt water. Hull must be perfectly clean of growth and props/lowers must also be perfectly clean. It only takes a very small amount of marine growth to massively degrade performance.
 
aliboy said:
Gulf Craft from Dubai? I used to sell them a few years ago. Was out in an old clients GC WA31 a few days back. If it is a Dubai GC it is likely a balsa core hull, so shouldn't be overly heavy unless water logged from some damage etc. From memory the GC WA31 will do over 30knts with a pair of 115's, so your speed is well down as are your WOT rpms. Most likely cause is a dirty hull if it lives in salt water. Hull must be perfectly clean of growth and props/lowers must also be perfectly clean. It only takes a very small amount of marine growth to massively degrade performance.
Click to expand...

Hi aliboy, this is a GC fisherman 2400, an old boat from 1996, yes the hull was dirty, now I am doing antifoiling and general maintenance, also propellers were bent.

I will do another test this week after maintenance and registration completes

Here are the photos attached

Thank you
 

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Thanks for the photos. With a bit of luck you will have fixed your problems with a clean hull and repaired props. Let us know how you get on please.
 
Engine maintenance is done, we went on test drive, max RPM was 4700, speed was 40km/h when propellers trim was completely down, we pushed it bit up and it made 50km/h at 5000rpm.

The technician mentioned something about the fins, he thinks the boat will run faster without the fins and suggested to remove them.

What do you say?
 

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Fins might be part of the problem, but I am not sure they will fix the problem when removed. With the engines vertical (parallel to the transom), how high is the cavitation plate when compared to the bottom of the hull directly in front of the engine? If the engines are set too low that could explain your speed issue and also why the fins were fitted in the first place.
 
The fins will definitely take away from your top end. kimcrwbr1 is correct, you will want to see 5500rpm. Reinstall the missing trim tab on the SB engine, take off the fins and retest.
 
Thank you everyone for trying to help, i have taken some pictures today after moving the trim down to full. Hope this help...
 

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Hard to see from the pictures, but the engines could be one hole to low. Hold a straight bit of wood or a long ruler etc along the hull (fore and aft direction) and check where that 'hits' the engine. It should be on, or below, the cavitation plate for most boats. Engine needs to be 'vertical' with the cavitation plate parallel to the hull bottom (it is trimmed under a little too far in the pictures).
 
I have removed the fins and tried without, no difference on speed, still max was 50km/h at 5k rpm max.Not sure if I will put them back or not.
 
You should still put a straight edge on the hull as I suggested above. If the engines are mounted one hole too low that will affect your speed and max rpms. The other way to check is to get the boat running in flat water at 4000rpm - 4200rpm and see whether the cavitation plate is just skimming the surface of the water. If it is, that is about right, but if it is under the water surface your engines are too low.

Assuming that this is a balsa core hull like the GC's I have dealt with, you should also 'tap test' the hull bottom to check for moisture in the core. You can do this using the back of a screwdriver handle or (best) a small hammer to go around the hull hitting it hard enough for the hammer to bounce a little. Good hull areas will make a 'sharp' sound and water logged areas will make a 'dull' sound. Water in a balsa core could give you lower performance as well as being a safety issue due to reduced strength in the hull.
 
I have placed the bar and found it touching the cavitation plate, I took a thick screwdriver and tried tapping, i heard tak tak, tog tog and tob tob on other places :D . I don't understand if this is normal or not, i asked my maintenance shop who serviced my boat earlier and said the boat is clean, no moisture inside.
 
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