Could you give me more info about my boat

Hello,

this is my first post on this forum, but I was reading it for quite some time and it was very useful when I needed some technical information and when I wanted to learn something new. Take into consideration that I am an electrical/electronic engineer, so please excuse my incompetence in naval design engineering, but I will try my best not to ask "stupid" questions.

Although speed is not my priority, this question(s) will be speed related.

I will use metric measures, just because less chance that I will make mistake buy conversions in some of data, this way it will be consistent.

In past two years, I have restored/renovated one fiberglass boat. Instead of giving text inputs here I have put most info on the photos because it is easier to perceive everything that way and it is, maybe, less boring. Please, disregard my good friend on photos, but pictures with white bottom, before painting it with primer and anti-fouling, give much better representation of bottom shape.

Info that is not on the photo, but is also of importance is:

1. Instead of two original gasoline engines from brochure, inside is now one Cummins V-504-M 195HP/3000RPM, Velvet drive (Borg Warner) AS72, with reduction 1:1 and stern drive is SternPowr in which is reduction 1:1.6, so that propeller is rotating 1.6 times slower than the engine. Propeller is DxP=19''x17'', BAR 65%. Engine room is now longer, since engine is longer.

2. Instead of two gasoline tanks, and one water tank in between them, now is only one fuel tank in the middle where water tank was and it has about 300 liters. Flexible water tank is now under the V-berth in the bow (260 liters).

There were more changes, but these two are major and they are made to compensate for the added weight of the "new" engine compared to old ones. And since all that weight is added aft, water tank is located far forward. It is not easy to calculate how much heavier is the boat now, but according to integrated equipment compared to old one and some calculations, my approximation is that boat is now between 3600kg and 4000kg, without people on board. Some non-conservative approximation is 4000kg with full tanks and 5 people on board, but I really cannot tell, and I believe that is maybe correct without cables, but believe me, those cables all together are heavy, so, let's just say this is some really approximate info on boat weight.

According to brochure, you can see the speed/power data, according to some calculations with new setup, calculated speed should be about 18-20knots with weight approximated to 4000kg.

What I get in practice on fresh water is:
1. 11km/h at about 33HP (I calculated this power from measuring fuel consumption). And I enjoy driving boat at this speed, and fuel consupmtion is great. This is displacement ride of course.
2. At displacement speeds I must make a lot of small corrections on the wheel while driving. On idling speeds, boats vary, so compensating is a bit more tiring process.
3. At full power, RPM goes a bit above rated 3000RPM, and the speed is 20km/h. I have tried using flaps, trim, everything, but that is just it. At that speed, there is a bit of water spray from the stern that reaches above over engine compartment. It seems like it is in planning, but the speed is not going up and like it is just at the speed barrier.

So questions:

1. Would you consider this boat as planning boat or semi-displacement, according to definitions, this is planning boat, if I have read all those books well, but that rather big keel is making me unsure? And then again, this varying of direction is typical for speed boats on small speed, but I thought that keel size is going to prevent that. I am a bit confused here.

2. Can the keel alter the water flow to propeller and at what degree? I have consulted the outdrive manufacturer (long story), and according to them, the keel ends at "enough" distance before the outdrive.

3. Could it be that propeller pitch should be greater?

4. Would you consider that weight is the main factor here?

I have put a couple of photos with waterline so you can compare, but all are not really from same perspective, so hard to tell, maybe if you look at the ratio between the curved bow part touching the water and the full length (since lines are probably painted different on each case).

I have probably forgotten to ask something and put some specification of importance, but I am sure that we can come up to that in following discussion.

I would appreciate any info I could get and learn from you. Thank you very much.

 

Well, it seems to me that you have a semi displacement hull but 195 HP isn't enough to get her over the bow wave and up on step a bit.

The 11 KPH speed you mention would be the maximum displacement speed for a boat with a 7.92 meter LWL (length water line). This is almost exactly the length shown in the line drawing you have posted there (8.02 meters).

You can push the boat to 20 KPH at full throttle but it seems based on your description that the boat is squatting at the stern and unable to get on plane due to insufficient power. Based on the brochure photos, it looks to me like a semi-displacement hull, even with twins. At 20 KPH you're well past displacement speed and I don't believe you could get to 20 KPH with a true planing hull. You would hit a speed wall well before 20 KPH.

I'm sure other members will weigh in. Nice job by the way.

MIA

 

It does appear to be more a semi-displacement hull, I am having trouble believing the figures given though, 20km/hr at 3000 rpm equates to an awful lot of prop slip for a 17" pitch prop driven through 1.6:1 reduction. The engine would not be able to do it, at least not without a lot of cavitation. If that boat can't get into the 30's (km/hr), the anchor must be out.

 

@Missinginaction, thank you. I went thousand times through Crouch formula, but I changed values a hundred times, and went through some online calculators, and some old rotating calculating tool from Cummins about horsepower/speed calculator, backward and forward always beeing unsure between planning and semi, and I always thought of boat as semi, but never 100% sure. The most confusing is that, according to original specifications that I gave in one of photos, boat speed was 50km/h (27 knots) with 300HP (twin gasolines), which is a lot compared to what I got.

@Mr Efficiency, that prop slip is something I doubt of, that is why I have asked about keel "removing" water from the propeller. When I look at the outdrive at 20km/h, only the propeller is in the water, cavitation plate looks like it is on the water surface (boat is much out of the water), but speed is still low. Then again, I use the flaps to put the bow down, but I do not have a feeling that I am planning, and at some point I have a feeling that it does start going a bit faster, but then it slips and does not gain acceleration and that happens a couple of times. Gearbox is new, therefore slipping is not from there. Engine acceleration is tremendous (RPM increases with ease), it is not like engine needs time to accelerate, rather I do it slowly to prevent some damage to the equipment. Speed was measured with GPS on the mobile phone, application called DigiHUD. Is there some practical way to check for that prop slip? What would you suggest? Experimenting with propellers is expensive sport, so I would like to be sure before taking that route.

Not so long ago, same boat (only difference is the hard roof) showed up on youtube with single more modern (less weight) diesel engine from VW with 265HP and although there is no speed shown, it does look impressive and seems to go faster. This proves to me that keel probably is not the culprit. Still not sure that 195HP is not enough to take me over the hump. Here is the link for that video:

https://www.youtube.com/watch?v=gBeAo4xwy44&spfreload=5

I must bring someone to take a video of my boat during the trial, but that will have to wait for the spring.

 

With enough power applied even a rock can reach planing speed.

Your 8m. boat with 4 tons and a keel is certainly not designed for that. With 350 HP and trim flaps she will be able to climb over the hump but steering with a long keel will be next to impossible.

 

@CDK, I do not have intention of being rude, but have you watched the video on the link I gave in the last reply, would you consider that speed over the hump and having trouble with steering (265HP, same boat)? The boat was originally manufactured, in serial production, as 27 knots boat with 300HP (therefore planning with much less than 350HP).

On the other hand, I do understand if you think that I have made a boat too heavy after alterations, but cannot agree that this boat is not meant for planning considering original design (truth, knowledge and boat design were different between '60s and '70s compared to today). Fiberglass itself is very thick and polyurethane foam core is being used. Modern boat of this type would be fairly lighter and on some you could even hear the "squeak" sound when walking aboard while the board would be flexing a bit. This older type is very stiff and heavy.

If it can help in some way, in the original user manual, which I do not have with me currently, there is a diagram of angle vs speed and the speed increase is almost linear in whole range, while "boat trim" angle was changing approximately from about 2.5 degree to maximum 5 or 6 degree and back to steady 3 degree (but I will scan this when possible, current values are from my head, so take this data with big reserve).

Anyway, being made for planning or not, I would like to have boat completely functional at all possible regimes and to know the limitations since I made it to here through whole restoration/renovation process, so if there is something that I am doing wrong I would like to find out. At the end of the day, I sail at 11KPH and the fuel consumption (6.3 liters/hour) at that speed is what I am ready to pay long term. As I said at the beginning, speed is not my priority, but would like to get through to the end. Considering that, long long term plan would be even to replace big engine for one smaller diesel that is even easier and cheaper to maintain, and probably fuel consumption would be even better (although that is not comparable to engine maintenance costs).

 

At that speed (20 km/h), power and rpm the propeller is very heavily loaded and cavitating a lot (over 50% from Burrill's chart) or the propeller is getting air. If you are getting over 3000 rpm, the power may be limited by the rpm limit.

For that kind of hull form there shouldn't be much of a hump. As you see from the video you linked the bow doesn't rise much and I would expect the resistance not to decrease at higher speeds.

When you went from twin to single and at the same time your boat got heavier and probably more weight at the stern, you need 2x or more thrust from a single propeller. What was the diameter of the propellers in twin setup? What size of propeller does the boat in the link have? Duoprop perhaps?

You would definetely benefit from a larger diameter and higher BAR%. At current size you have a lot of cavitation, slip and very poor efficiency. If you can't increase the diameter (like it looks like in the pictures you included), at least increase BAR% (maybe 4 or 5 blades?) and also it may help to have a propeller better designed to deal with cavitation. The propeller you have in the pictures doesn't probably deal well with cavitation.

Increasing the pitch is not going to be a good solution. Your current pitch should be fine up to about 20 knots, if you just can get rid of excess cavitation/ventilation.

The "cavitation" plate doesn't need to be under the water, but it needs to touch the water. In this case it may help to have it under the water as well.

 

Nice boat really, almost no "hump" in the video boat. Your boat should be peaking near 20 knots, unless there is a big weight problem. I can't credit that you are topping out at 11 knots at full throttle, 3000 rpm, with that engine, drive, and prop, there is something amiss with the numbers quoted earlier.

 

@Joakim, that is very interesting and informative, thank you. It does make sense, especially since some water spray reaches over the engine compartment on the deck. This propeller was calculated by the outdrive manufacturer and they were concerned about the boat weight and non-standard hull shape, therefore it is not their standard propeller (greater BAR than standard), and it is considered as heavy load propeller, estimated speed was 18-20 knots, as you have said. And yes, the outdrive has 2 inches extension to make it deeper and to correlate with boat bottom and keel. I will have to take one more trial and take a video so I could tell exactly where the cavitation plate is at that maximum speed point.

What comes to mind, I am not sure if that can be seen on that photo from the stern that I have sent, beside added controllable trim tabs on port and starboard side, there is also fixed trim tab in the middle, which "makes the boat" longer, and it is even bolted to the outdrive housing bell. Could it be that this fixed tab is preventing significant amount of water reaching the prop?

 

Does the engine have a rev limiter ? Is the propellor SS or alloy ?

 

I am not really sure if the engine has that governor limiter, since it is mechanic engine, but according to RPM gauge it goes above 3000 RPM (and gauge was calibrated, therefore, it is truth what it is showing), so I assume that it does not have limiter.

Prop is alloy (aluminum).

 

It's hard to see from your pictures, but the tab in front of the drive should not be lower thant the "cavitation" plate when the drive is in its normal trim.

When were all these tabs added?

I wouldn't be happy at 11 km/h and 6.3 l/h. Just look at a test of a planing boat at similar size and weight with VP D6-370 hp diesel. It consumes only 3.4 l/h at 6.0 knots (11.1 km/h). And a slightly bigger one (9.7 m 5 tonnes) with the same D6-370 4.2 l/h at 6.2 knots.

 

Any suspicion of cavitation/ventilation problems, change to stainless. Expensive of course. Maybe cupped stainless as well. I have had boats with persistent problems of the kind, solved by switching to stainless. It could be with a rev limited engine, you don't notice the cavitation, which may be caused by that big skeg ahead of your drive.. Stainless gives a lot more latitude, with thinner blade sections that don't let go so easily. I am not seeing this middle tab you speak of ?

 

@Joakim, I have forgotten to add, this is the maximum diameter prop that can fit on this outdrive (19''). So, as you said, I could experiment only with BAR, number of blades and different prop shapes.

 

Governor will most like have a rpm limit. You can test that on idle. You probably won't get more than ~3200 rpm and the maximum fuel supply will be limited very soon above 3000 rpm.