Keel modifications

Thanks again for you replies. I am pretty certain about my top speed, I have 2 gps-plotters on board. I will try and get a photo of my boat underway to have an idea about the trim angle. Playing around with the proc-calc site, I entered the 5 different ratios available for the borg-warner, they all came up with a top speed of 12.42knts with prop diameters from 15--28" I will probably go somewhere in the middle. One thing I noticed was 3 of the calculations called for 4 bladed props and two for 5 bladed ones. Any idea why a three bladed one cannot be used? Finding this an interesting project--lots of questions unasked yet!!!
Rammy

 

Basically because you have such a low g/box ratio. It is generally bad practice to use a high rotating prop to drive a slow boat. The greater number of blades help to increase the BAR (blade area ratio) and delays cavitation; it reduces the thrust loading on each blade.

 

The highest ratio I tried was 3.4 this called for a 28x24" prop with 5 blades. Is the perkins 2800 a high rpm or about average?

 

I don't know what input the program uses, but the numbers are all wrong. That is the program with these "simple DIY" prop design programs....gives you answers, but not real ones, since it doesnt know what a real answer is!

You have an rpm of some 2300-ish...at 120-ish hp...the diameter prop best suited to this is roughly 16", with a pitch around 7~8". This is based upon the data you have provided. As i noted above, your boat is slow and your shaft rpm is high..bad combination!

When you says the highest ratio of 3.4...ratio of what?..this is not clear.

 

Sorry, I made a typo there. The ratio I entered from the borg-warner book was. 3.14: 1.00
The other ratios are
1.99:1.0
1.51:1.0
2.49:1.0
1.21:1.0
0.96:1.0

The nearest to your calculation it came up with was a 15x9 prop with 5 blades a DAR of 88% a slip of 0.31 pitch ratio 0.60. This was with my current ratio of 1.21:1.0.
Intresting comment about the calculation site, how else can I go about these calcs?
Rammy

 

Well, ask yourself this question...How many boats of your size/type/power have props of 28x24 going at 12+knots?

Whilst we can debate maybe the pitch of diameter should be slighter higher or lower, accordingly to the calc's....but, is it a realistic meaningful result? Ergo, how many boats do you see with a 28x24 prop?

Naval Architecture (designing boats) is not an exact science. There are disciplines within, which are such as: mechanical, electrical and structural, some aspects of hydrodynamics too. However, as a complete package, the "exactness" often falls down in reality. ..ie a boat is greater than the sum of its individual parts. Only experience can tell you this, not numbers spued from a program. In the absence of experience, look around you and your marina/magazines etc..what do you see, hence the question about how many props of 28x24 do you see? So, some self analysis can be done to obtain a better idea.

So, a 28" prop is very very large for a boat your size and power, just go to your local marina and check.

There are only 3 things to consider when selecting a prop
1) The speed of water in way of the prop (different for different hulls)
2) Horse Power at the prop
3) Prop RPM.

The hull, can only go so fast with so much power given and also the shape of the hull shall dictate the speed too...etc etc...so, many things to consider, which a prop DIY program wont tell you.

PS..forgot to add.
If your engine has an even number of cylinders, ie 2,4, or 6 cylinders, for example, best to select a prop with odd number of blades. And vice versa.

 

Thanks for the detailed reply, starting to make sense of it all now, I think! I have tried to contact similar boat owners with limited results. Most have bigger engines and set up with a leg, which I do not want. One I know of has 175bhp with a 14" prop and achieves 14knts. Not very efficient methinks. I appreciate it is not an exact science, like my trade joinery, experience is crucial for a good outcome. The perkins has 6 cylinders. My choice is a 16x8-9" 3 bladed prop. may get one off the shelf £2-300 or a custom 5 bladed one, considerably more I imagine. I won"t hold you to it, but which would you chose? Hey I"m getting a collection of props now, so one more won"t hurt!!
Rammy

 

Before you start selecting your prop, your original question was about just simply selecting a bigger prop.

So, firstly, back to the beginning..why do you want a bigger prop?

Is it simply because the current performance of your boat is less than expected?..or is there another reason?

Just want to establish the basic facts first. Since this thread has deviated a tad and not sure what you really want as your priority.

 

I just want a bit more speed, I fish offshore at times and the journey time can be a chore. 14.5" is the maximum I can swing at the moment, and it just (looks wrong). I love this old boat and will re-engine with more horse-power (which I assume will need a bigger prop) when funds allow. I am pretty practical, and my son is an chief engineer on a super-yatch so hope to do most of the work between us. That"s about it, I enjoy boating and solving boaty problems.
Rammy

 

Ok, so it come down to some basics.

Your hull has a hp/ton around 24, this should get you around 11~12 knots may be a tad more. However, the length displacement ratio of just over 4. This is low.
The L/B ratio is also low...all in all, which means even though the Froude number (at 12knots) is around 0.7, the hull is struggling to go faster. So even though the hp/ton suggests a predicted speed, doesn't always mean it can be achieved, owing to that already mentioned.

However, this and the following really depends upon 3 factors
1) Is the displacement correct?...how ahve you checked it to confirm the 5tonne?
2) Is the Hp correct, again, how can you confirm this is correct?
3) Is the RPM correct, again, how can you confirm this is correct?

In each case, you need actual figures, not figures taken or given from manufactures. You need to actually measure it.

Because the performance of your boat AND the prop is governed by the above. Any slight deviation, say 100Hp and not 120HP, will effect the selection.

The main "issue" is that your engine power is high for such a small heavy boat, which suggests, as noted, the hull parameters are not desirable for going fast. This is not to say you can't, since you ahve a "chined" hull...but it is really struggling.

 

Hello again, The last time I had boat lifted, the hoist operator said between 4+ 5 tons. I opted for 5 tons to allow for fuel, water and passengers.
The horsepower is only the given one in the manual, compression tests were done and ok, engine runs and sounds ok. Can HP be tested away from the bench?
The RPM of engine and shaft were checked with an optical tachometer.
The boats tachometer is a new Vetus and confirms the optical taco for engine RPM.

 

Hmmm...4 or 5 tonnes...that is a difference of 20%!!!!, on any boat, this is not a small amount...you really need to establish the lightship and then what is your max deadweight to get an accurate full load.

 

Don"t really know where to get an accurate weight, there is a crane on a river nearby, but he can only set his jib alarm, so not the most accurate I would think. The 40ton hoist operator in my harbour says he cannot give a totally accurate weight due to the configuration of the sliding strop mechanism. This has 4 weight dials on it, maybe he just doesn"t know to use it. So I am a bit stumped there. How about the HP issue, any thoughts on that without removing the engine for testing?

 

Well, a quick way to establish the displacement(or mass), although not wholly accurate on small boats, is to read the draft marks and check against the hydrostatics of the stability book. This assumes of course you ahve a stability book? You would need a hydrometer to confirm the salinity of the water too.

Do you have the engine performance curves, from the manufacturer??

 

I do not have a stability book.
I do have the performance curves.