View Full Version : ..anyone help?

erbus - n/a
27-Sep-08, 08:53 PM
...went out today for the first time in a good while the refubished craft but came across a few problems that somone may be able to shed any light on:

the craft is used on the sea and it was about a seastate 0-1 in the bay and about 1 outside shelter...and breezy.

i upgraded the blade number from 6 to 9 after having problems with lift last year it wouldnt rev fully but thats probably the blades nedding backing off a few more degrees.

the problem i have is the front bags seem poorly inflated and easy to collapse.

i seem to be losing a lot of lift air along both sides about a third ways back from the front.

i seem to have to rev quite highly about 5000 just to get lift (cuyuna 440, dont know gearbox ratio)

its a old ghl dragonfly integrated hull.

(from photo, most air is lost from 1 seg fwd of windscreen and about 3 back - that sort of area)

it was a bit rough on the open sea at times, which may have been disrupting things a bit.

does the revs for lift seem normal?

from anyones experience, how badly does wave action effect using the craft at sea?

i can fix most things but its the heavy air leakage on the sides coupled with poor front bags (all the segments are open)

any help would be very much appreciated and it was good to get going again - kids loved the soaking!

Ian Brooks - n/a
28-Sep-08, 10:38 PM

We're going to need a bit more information I think. You'll need to find out what the gear ratio is - perhaps manually turn engine over say 5 times, and count how many blades go past. Then let us know the numbers and we can work out the ratio.

There are a number of things that could be wrong to give poor lift, but first it's useful to understand whether the set-up "should" work. If you can also let me know the craft length, width and weight, then I can work out the engine revs at which the first signs of lift should occur. You should have better lift with 9 blades compared to 6!

One thing: At what speed are you losing lift? Is it at low speeds (around 4-5 mph)?



erbus - n/a
29-Sep-08, 08:15 PM

thanks for that.

the fan turns 1 for every rotation of the engine - therfore ratio approx 1:4

length=3.3m, width=1.8m blade angle at 37 only let me rev to about 6k (max rpm 7k), now set to 35 degrees but untested.

wieght unknown, i have no easy way of wieghing the craft short of hauling into work and putting on a load cell but that would be a h&s nightmare.....

the lift is a strange one, its not that it is not lifting i'm just losing what appears to be an excessive amount of lift air thru the areas outlined.

having had good advice it maywell be the skirt segments are not packed close enough and also thier age (prob about 15-20 yr old) leading to degraded material.


Ian Brooks - n/a
29-Sep-08, 08:44 PM

To weigh it you need two bits of 4"x2" about 8 ft long; 4 bricks; 1 set of bathroom scales; 1 pencil; 1 piece of paper; 1 person who can add up; 2 strong people!

On a good level surface, put the 2 bits of wood under the craft at the front and rear. Put a brick under each end of the wood so they are clear of the ground.

At each end of each piece of wood, remove the brick and substitute the bathroom scales, using a block of wood to make sure the scales are the same height as the brick. Note down the weight of that corner, repeat for each corner and add up.

the fan turns 1 for every rotation of the engine - therfore ratio approx 1:4

Is there a typo here?

When you see the lift escaping, how fast are you going?



PS - weigh yourself at the same time... you can PM that info if its sensitive!!!!

erbus - n/a
29-Sep-08, 09:12 PM
...def a typo, engine rotates 4 times for single rotation of fan..

will have to wait for weekend for wieght, although its pretty heavy by todays standards as the lower hull has had two more fibreglass layers added, it takes 25l of fuel in the tunnel tank (empties it pretty quick so suppose its a moot point!)

I also tend to carry quite a lot of gear as its used at sea.

the lift air is lost from when i'm stationary, its tends to reduce over the hump but still blows from the same area (we always get soaked and the engine gets upset with the fuel/air/saltwater mix!) - it also gets bad when crossing over small waves.

just out of interest, i expect choppy seas will upset the cushion but is this in relation to my cushion hieght? I believe the factory spec on this craft was originally 8".