Beyond 12 Knots

[dan4836]

Looking for some info and help regarding speed of the 27 Sport Cruiser.

I know these boats are semi-displacement / semi-planning but I have a much larger engine and should be able to cruise at 12 knots to 14 knots.

I have a 7.3 International engine, 270 HP, turbo intercooled but I can only get 12 knots at 3800 RPM (max WOT) I am running a 17" X 15" four blade prop with no cup and 1.5 /1 transmission reduction. I know the I am over turning the prop because I can exceed my max WOT.

At 3200 RPM's I was cruising at 10 knots for a few hours and around two miles per gallon. I am setting this boat up for fishing and speed is more important than fuel economy.

Some have put the Cummins 6BTA at 210 HP and can cruise at 14 knots. I am not sure why I can't get this with 270 HP. All prop computers say I am good at 17 X 15. Can I be seeing so much prop slip due to cavitation? Or, is the prop too close to the keel? (I have about 3.5" between the prop and the keel)

Thanks,

Dan

[Mark Deeser]

Dan, are you sure the 27 is the boat for what you are looking to achieve ? Would seem a 28 would be a better fit, just saying. Thanks, Mark.

[dan4836]

28 TE would be a better fit but I got a great deal on the 27. I love the way the boat handles with the large engine down low and the space would make a great fishing platform. I must have some type of prop issues, cavitation or need more pitch. One person mentioned I should try a three blade prop instead of a four blade because I might be creating more turbulence and cavitation with the four blade.

[don123]

I'm no expert, but I would guess you just need more prop - I'm using a 17 X 12 3 blade with a 100 HP Yanmar with a 2:1 reduction. It just seems 270 HP should need more wheel

I would call a technician at Michigan Wheel and discuss your boat with him. I did this when I repowered and got some great advice

Don

[jleonard]

I agree you need more prop and a prop shop should be able to dial it in for you.
You say you are exceeding max WOT rpm. Are you at your "High Idle" or unloaded WOT rpm?
Can you go with a larger diameter prop? You are close to "square" and if you go to much more pitch than diameter you may "stall" the prop.

[sail149]

Hi
The 1993 boats with a 180hp mercuries ear/BMW engine has an extra wood strake added at the turn of the bilge aft just under water. Might help with water flow and tricking the hull into a more semi planning hull.
I have a photo if you would like to see.
Also do you have the boat empty of extra weight?

[dan4836]

Thanks for all your responses. I am going to try more pitch with a cup to see if I can bring my RPM's down to max WOT 3800. I think this will give me a few extra knots that I am looking for to take me from 10 to 13 - 14 cruising speed.

Anyone on this forum have the Volvo TAMD 200 hp or the Cummins 6bta at 210 HP?

My last fishing trip in the ocean, I was at 3400 RPM and averaging 11 knots. The boat seemed to plane or semi plane at 11 knots and run well. Down seas, I would hit close to 13 knots with little stern wake and everything felt great. Cruising at 14 knots would be ideal.

[dasabalon]

Hi I have a 1987 Sport Cruiser with a 157 isuzu with mechanical hurth gear 1.5 shaft and A 4 bladed wheel 18x18 I cruise at 2700 Rpm 12.5 knots wot 3000 because of the wheel size gets me to 14.9. 3 gallons an hour average sometimes less sometimes more like 3.5 if I run her hard. most of the time alot less.

[dan4836]

Dasabalan - Your numbers are what I was basing my numbers on. If you have 157 HP and can get almost 15 knots WOT, I should be able to cruise at 15 knots with 270 HP around 3000 RPM's I am going to try a much heavier pitch prop to see if I can get closer to these figures.

As of now, 3,200 RPM's only gets me 11 knots. I think the prop is either severely under pitched or I am experiencing severe cavitation.

[SkipRocks]

Dan,
I have the 150 hp Volvo/Penta TAMD31 in my '85 Albin A27 Express or Sport Cruiser (no aft cabin). The TAMD31 is a 4 cyl (the TAMD41 is the 6 cyl). No idea what size, but it's a 4 blade prop. At 2700 it runs at 10.5 kts. 3000 = 12.5 kts. 3400 = 12.51 kts.

I've started a log on speed/RPM/fuel burn. The speed/fuel burn doesn't have very many accurate data points because I need slack tide to get an accurate read. I must take 6 hour trips, because I usually seem to go against the tide both ways on the intercoastal. What I do know is that fuel burn at 2700 is a rock steady 3 gal/hr.

I brought the boat up from Fort Lauderdale to St Augustine FL in June and took advantage of the Gulf stream (3.9 to 4.2 knots due North directly off of Fort Lauderdale when floating dead in the water with no wind). I have a picture of the plotter showing the Albin doing 16.7 knots.

Were you to ask where I prefer to run it, that's at 10 kts or less. I don't have to scream to be heard and the deck isn't tilted up as much. No planing on this Albin :-!

[dan4836]

Thanks for the info.

I found one of my problems! I am taking my RPM's off of my alternator because I could not find any other ports on this motor to use. I purchased a Faria variable ratio tachometer that uses a port on the tach for input. After checking the tach accuracy against a photo laser gauge off of the main pulley, I found my setting was off.

Now at 3000 RPM's, I am running at 14 knots at 3200, I am at 15.5

My max rpm's is 3800 and I did not push it to max throttle yet. This will be my next test to see if the I can turn the max rpm or if I need more pitch.

Even at this rpm, the engine does not feel like it is loaded up. I still think there is cavitation or I need more pitch or some cup. I think with the right prop, I should be closer to a solid 16 knots at 3000 rpms but I could be wrong because of the shape and big keel drag.

The boat plans great and moves very well through the seas.

My last concern is pushing this boat this fast through heavy seas could be a another problem because I know the hull was not built for it.

Has anyone filed the keel with Seacast or Arjay? I think this would strengthen the keel.

Thanks,

Dan

[sail149]

Dan
You don't say what age the boat was. 85 was first sport cruiser I think. What was original engine? My 84 FC HAS A VERY THIN KEEL, sides are barely 1/8". I assume they beefed thinks up when they it in the bigger engines.
I would have a look at what the water is doing around the stern and sides. Make sure it's not squatting too much and causing drag. The rounded hull sides are not good for planning and the water can creep up and you loose lift , creat drag and go slow. You really want to see the water squirting out sideways toward the back of the boat.
An underwater or waterline strake on the aft sides may be what you need.
Warren

[dan4836]

Warren,

Thanks for the response. It is very thin around the keel area and I was thinking of filling the keel with Arjay or other fiberglass type filler to strengthen things up. I know this boat was not designed to do the things that I am trying to achieve but the layout works very well for fishing. In fact, there are two guys using this model for charter fishing. Also, I may add some stringer systems under the floor to make sure the boat can hold up in rough conditions at 15 knots.

[Beta Don]

I have a 7.3 International engine, 270 HP, turbo intercooled but I can only get 12 knots at 3800 RPM (max WOT) I am running a 17" X 15" four blade prop with no cup and 1.5 /1 transmission reduction.

You are trying to couple 270 HP to the water with a 1.5 reduction trans with a 17 X 15 four blade prop

I have a 1987 Sport Cruiser with a 157 isuzu with mechanical hurth gear 1.5 shaft and A 4 bladed wheel 18x18 I cruise at 2700 Rpm 12.5 knots wot 3000 because of the wheel size gets me to 14.9. 3 gallons an hour average

He is running even more prop than you (18 X 18 four blade, 1.5 reduction) and it's getting him 15 knots with less than 157 Hp. He may be a bit over propped since he can't reach max RPM's, but he's only got 157 Hp. If he had 200 Hp, his 18 X 18 would probably be prefect

I think there is little question that you are way under propped at 17 X 15 and at least 100 of your 270 Hp isn't getting coupled to the water, since your maximum speeds are very similar to his, and he's doing it with *less* than 157 Hp since he can only make 3000 RPM's. I think if you had an 18 X 20 four blade, your big motor could spin that very close to maximum RPM's and you might actually be looking at something close to 20 knots . . . . assuming fuel burn isn't a big issue

Don

[sail149]

I think I would certainly consider reinforcing the hull espically if it's an early one orriginally fitted with a much smaller engine .
You will of course want to get it running the way you want first before you go to the trouble.
Arjay or similiar products may be the way to go. But getting the right people to do the work if you are not skilled yourself is the most important. I was told after I bought mine buy the boatyard it was stored at that they had done some major repairs to replaced failed balsa coring in the stern underwater area! Fortunate they specialized in fiberglass repairs so it had a good job done on it.
Warren