Does everyone lie about boat speed?

B

Barleycorn

Well-Known Member
Joined
22 May 2013
Messages
158
Visit site
I bought an Aquastar 27 a couple of years back, and on the whole I am very pleased with her. The previous owner advertised that she would cruise at 12 knots, and top out at 17. She has a Cummins 210 HP engine. However at 1900 revs, which I would think is a good cruising speed she barely makes 8 1/2 knots, and does 10 flat out, which is 2300 revs, although the engine is rated to run to 2600. Paul Bell, from First Marine Propulsion, (an excellent firm to deal with), has done prop calculations and she is fitted with a brand new 19 x 19 propeller. The bottom is clean, as I keep her on a drystack. I have looked at adverts for similar boats, and ones with the same horsepower quote 10 to 12 knots cruising, 15ish tops.
Is it an issue that my engine doesn't rev to max speed? Could it be down on power? It's only done just over 1000 hours. I am wondering if I should have the pump and injectors serviced?
 
I don't think people generically lie about speed, but tend to quote the manufacturers numbers from the boats original launch, part fuel, no water, no "stuff" on board etc.
The quoted max speed for my boat is 32 kn, but on her sea trial I was happy to see 29, given we were 6 up, half tanks, and a few years of accumulated stuff.
 
How heavily loaded is she ? My last boat was 28' on a VP TMD40 140 HP and She would do the speeds you report.

I think many speed claims assume an empty fresh water tank and just about enough fuel to leave the mooring, open up to full power for 5 minutes then get back in again.

My Turbo 36 with the TAMD60C was claimed to do 25-27 knots, but that must have been downhill with the wind behind her. Early season on 1/4 > 1/2 tanks she would just about make 23 knots. I suppose if I had stripped her bare with just me on board she would have likely hit the 25knots.

Annoying for you though as I suspect you are not getting over the semi-planing hump, which is not fuel efficient either. I would have expected you should make 15 knots. Possibly slightly over propped as you need to use those last 300 revs. I would try a slightly finer pitch, say 17.5 - 18".
 
We have a mitchell 31 MKIII, single Perkins sabre 225, semi displacement hull and heavy layup so as longer and heavier compared to your stats we should be slower, but we aren't. we cruise at between 2000rpm which is 10.5knots and 2100rpm which in smooth seas gives just under 12 knots, WOT is 2500rpm 15knots. When we did the sea trial with full fuel tank, empty water tank & 4 adults on board she was 1 knot faster WOT. Now We have a dinghy, 2 outboards, kayak & general clutter on board and 6 months of growth on the bottom and even yesterday with roughish conditions and 4 adults and one child aboard 2000rpm gave 10.5 knots

I would say your engine or prop is not producing what it should, Do you have any vibration at all? Could your prop have been dinged? Does the turbo need refurbishing?
Latestarter1 is the cummins expert, I am sure he will have some suggestions.
 
My boat has a scary fast top speed.

On a calm morning, w/ flat water, a clean bottom and clean running gear, my boat will do about 700 MPH, if dropped from an airplane.
 
Barleycorn said:
Is it an issue that my engine doesn't rev to max speed? Could it be down on power? It's only done just over 1000 hours. I am wondering if I should have the pump and injectors serviced?
Click to expand...

Does the engine rev to 2600rpm or beyond out of gear? If not you've got some kind of engine related problem and you should have the engine checked out by an experienced mechanic. If the engine does rev to 2600rpm or beyond out of gear then it is either a prop problem or the engine simply isn't making its rated power to drive the prop to the engine's rated max rpm of 2600rpm

First check the prop is in good condition ie clean and without any dings in it. Do what you can to lighten the boat ie take the dinghy off, empty the water and holding tanks and run the fuel tank down to near empty. You should see a rise in engine rpm and boat speed. If that happens, it confirms that either the engine is down on power or the prop is not correctly sized for the power/rpm available on your particular boat. Yes, a prop calculation will give you an idea of the correct size of prop for your boat but it is an indication only and you may have to fiddle about with prop sizes to fine tune speed and rpm

If you're satisfied that the existing prop is in good order and it is indeed the 19x19 prop that the calc suggests, get the engine thoroughly checked out including a compression test and fuel pump/injector service. If the engine is OK, then you're going to have to start fine tuning the prop to get those engine rpm up to 2600rpm or a bit more
 
Potential idiot noob question here but.....

Coming from a 20 foot cuddy I was initially dismayed that the Formula 34 PC I have recently aquired seemed to only get to the upper mid 20's in what I'd consider gentle seas (0.5 m swell 7 sec period) and rather lightly loaded. When on the Menai in completely flat water and heavily loaded. Full fuel, water and 5 adults , 4 kids on board she topped out in the high 30's (once 32 mph was hit it was like a turbo boost kicked in she just started accelerating at a phenomenal rate) and if I hadn't hauled back would have kept going. I know the boat is missing a lot of original weight (generators, icemakers, inbuilt vacumm cleaners etc) all located originally under the cockpit where the bods were sitting. Could this have an influence or are the light seas capable of soaking up so much power?

Edit - in the smaller cuddy power was never an issue, you usually hauled back on the throttles to stop slamming so the above was never really identified. The 34 doesn't slam or porpoise in these conditions, but the cuddy could.
 
Last edited:
BruceK said:
Potential idiot noob question here but.....

Coming from a 20 foot cuddy I was initially dismayed that the Formula 34 PC I have recently aquired seemed to only get to the upper mid 20's in what I'd consider gentle seas (0.5 m swell 7 sec period) and rather lightly loaded. When on the Menai in completely flat water and heavily loaded. Full fuel, water and 5 adults , 4 kids on board she topped out in the high 30's (once 32 mph was hit it was like a turbo boost kicked in she just started accelerating at a phenomenal rate) and if I hadn't hauled back would have kept going. I know the boat is missing a lot of original weight (generators, icemakers, inbuilt vacumm cleaners etc) all located originally under the cockpit where the bods were sitting. Could this have an influence or are the light seas capable of soaking up so much power?

Edit - in the smaller cuddy power was never an issue, you usually hauled back on the throttles to stop slamming so the above was never really identified. The 34 doesn't slam or porpoise in these conditions, but the cuddy could.
Click to expand...
what speed was that crazy barstued doin out at sea u were following
 
BruceK said:
Potential idiot noob question here but.....

Coming from a 20 foot cuddy I was initially dismayed that the Formula 34 PC I have recently aquired seemed to only get to the upper mid 20's in what I'd consider gentle seas (0.5 m swell 7 sec period) and rather lightly loaded. When on the Menai in completely flat water and heavily loaded. Full fuel, water and 5 adults , 4 kids on board she topped out in the high 30's (once 32 mph was hit it was like a turbo boost kicked in she just started accelerating at a phenomenal rate) and if I hadn't hauled back would have kept going. I know the boat is missing a lot of original weight (generators, icemakers, inbuilt vacumm cleaners etc) all located originally under the cockpit where the bods were sitting. Could this have an influence or are the light seas capable of soaking up so much power?

Edit - in the smaller cuddy power was never an issue, you usually hauled back on the throttles to stop slamming so the above was never really identified. The 34 doesn't slam or porpoise in these conditions, but the cuddy could.
Click to expand...

I'm a bit confused by your post, are you saying that the only difference between when the boat would only do upper mid 20 mph and when it would do 32+mph was the sea conditions? or are you saying it was substantially lighter when it would do 32+?

The difference between flat calm and a gentle sea wont knock much off your speed in my experience, and the more loaded up the boat is the slower it will go. The main thing that will drop your speed is fouling on the hull, a small amount of greenery will knock a lot off your speed. other factors that will significantly affect your top speed are tide direction and speed, wind direction & speed.
 
Apologies if I was unclear. The boat went much faster when heavily loaded but in calm flat water. Slower when lightly loaded in calmish seas. It seems contrary to all expectations.
I asked because she is missing at least 300kg of weight from her port side, a third forward of her length from the stern, where a onboard genny, central vacuum cleaner, heater and ice maker used to sit. Has loosing this weight affected her trim such that when she was heavily loaded it brought her back to a designed loading and thus a improved performance,? Because when there is just two adults onboard albeit at sea rather than flat calm we lose plus10mph. Very odd. I wanted to replace lost ballast as the boat has a light list to starboard but was advised to keep the boat light but try reposition house batteries etc.
My question was would a small swell of 0.5m cause such a loss in performance or would the loss of ballast alter the planing dynamics so much that is what accounts for the performance drop?
 
Barleycorn said:
I bought an Aquastar 27 a couple of years back, and on the whole I am very pleased with her. The previous owner advertised that she would cruise at 12 knots, and top out at 17. She has a Cummins 210 HP engine. However at 1900 revs, which I would think is a good cruising speed she barely makes 8 1/2 knots, and does 10 flat out, which is 2300 revs, although the engine is rated to run to 2600. Paul Bell, from First Marine Propulsion, (an excellent firm to deal with), has done prop calculations and she is fitted with a brand new 19 x 19 propeller. The bottom is clean, as I keep her on a drystack. I have looked at adverts for similar boats, and ones with the same horsepower quote 10 to 12 knots cruising, 15ish tops.
Is it an issue that my engine doesn't rev to max speed? Could it be down on power? It's only done just over 1000 hours. I am wondering if I should have the pump and injectors serviced?
Click to expand...

Barleycorn if you are only making 2,300 rpm you are WAY over-propped, perhaps Paul Bell is not as good as you thought!! Running in this condition is an engine killer.

Forget pump and injectors. Unless you have passed contamination they will last the life of the engine.

Been plenty of good tips here, but lets break the job down into bite size chunks.

# Purchase a digital non contact tacho off Ebay, cheap as chips.

# Using the above check the accuracy of your VDO tacho, should have been calibrated at new engine sign off however you MUST verify.

#Get boat in the water and take her for a spin, warm engine through nicely. With thermostat open engine out of gear perform high idle test, open throttle wide and hold it there, let engine sing until you are 100% sure revs have stabilised. High idle spec for B 210 is 2,808 low to 2,912 rpm high. If you re NOT making CALBRATED numbers in this range start again. Slip Morse control ball joint off the lever on the injector pump repeat above but operating throttle lever by hand.

Likely Morse cable adjustment is short changing you so adjust cable until it makes the same high idle number as it did with cable disconnected.

Assuming all above checks out, allowing for the fact you are a dry stack boat I want to see 2,700 WOT rpm on your calibrated tacho with half tanks and 3 up. There is no nearly, it is a must.

Unless you can make the above tell Mr Bell to poke his prop where the sun don't shine unless he graunches some pitch off!

Give me accurate weigh off the boat lift together with your transmission reduction and I will do calculation for you.

Good luck
 
I took a quick SWAG at the problem.

Assuming you have LWL of 24 ft a ZF 63A with 1.56:1 ratio based on choice of 19x19 prop and a boat lift weight of 3 1/2 tonnes you are at the semi displacement transitional speed at around 11 knots pulling about 80 hp out of the engine based on a factor of 3.

Suggest you complete first steps and give us some accurate data and we can progress.
 
I also like to keep things simple ...., but I would also advise OP to look at the fuel filters & tank uptake ... Clogged fuel filters (primary & secondary) will restrict fuel flow and thus RPM when under load (may not be apparent when in neutral), then I'd follow Latestarter's advise....
 
Barleycorn said:
I bought an Aquastar 27 a couple of years back, and on the whole I am very pleased with her. The previous owner advertised that she would cruise at 12 knots, and top out at 17. She has a Cummins 210 HP engine. However at 1900 revs, which I would think is a good cruising speed she barely makes 8 1/2 knots, and does 10 flat out, which is 2300 revs, although the engine is rated to run to 2600. Paul Bell, from First Marine Propulsion, (an excellent firm to deal with), has done prop calculations and she is fitted with a brand new 19 x 19 propeller. The bottom is clean, as I keep her on a drystack. I have looked at adverts for similar boats, and ones with the same horsepower quote 10 to 12 knots cruising, 15ish tops.
Is it an issue that my engine doesn't rev to max speed? Could it be down on power? It's only done just over 1000 hours. I am wondering if I should have the pump and injectors serviced?
Click to expand...

I suspect you've got the wrong prop. I had a CI32 with a 210 hp single engine and I got about 18kts top speed with cruising between 12-14kts. This was with a 4 blade prop, although I now can't remember the size of it, but pretty sure it was nothing like 19 x 19.

I also happen to co own an Aquastar 27, my co owner has just ordered a new prop for her, I'll find out what he's gone for and report back. Even with a smaller engine ours is faster than yours.

The new prop is 19 x 14 4 blade on a 120hp engine. Not got it yet so can't report as to performance, but as my co owner is a naval architect I suspect he will have got it right!
 
Last edited:
Thanks for all the suggestions, you all are really helpful.

Latestarter the gearbox is a ZF Type IRM 220A, 1.533/1, she is 24' at the waterline, but weighs nearer to 5 tonnes on the forklift.

To be fair The new 19X19 prop was given to me by the previous owner as the one fitted was a bit battered, and Paul Bell did say whilst not being ideal, he thought that it would do the job.
 
Portofino said:
Same as everybody else's then !:)
Click to expand...
Nope..
Sports boats fall faster than Flybridges which are considerably faster in free fall than rigged sailing boats.

Terminal velocity is about 200 km/h (120 mph). Stable freefall head down position has a terminal speed of 240–290 km/h (around 150–180 mph). Further minimization of drag by streamlining allows for speeds in the vicinity of 480 km/h (300 mph).

Somebody else telling porkys' about speed eh?

RR
 
You must log in or register to reply here.

Other threads that may be of interest

Refueler
Darn it ...... bl***y engine wont start - want to lift out boat.
2
Replies
34
Views
2K
Refueler
Refueler
geem
Atlantic crossing 2025
2
Replies
39
Views
1K
geem
geem

Share this page

Top