engine sizing for an F28 ck Hoyt

Engines, Drive trains, Propellers, Steering, Ground Tackle and other mechanical system

engine sizing for an F28 ck Hoyt

Postby seadago » Wed Sep 25, 2019 7:19 pm

Nausikaa came originally with a Volvo MD7A 12 HP. As that engine kept giving me no end of trouble, I replaced it with a Beta 16 HP. Following the 4 HP/tonne rule, and with a nominal displacement of just over 3 tonnes (maybe 4 t when fully loaded), I figured it would be just the right amount of power. It was also the largest Beta engine that I could comfortably fit into the bay.

However, on a recent extended cruise down from Lowestoft, Enland to Brest, France (that is down the North Sea and English Channel), the boat felt significantly underpowered. Some of this is certainly caused by the wrong choice of prop, but that is a known factor which I am soon to remedy. Still, in perfect conditions; no wind and flat seas, I struggled to maintain 4.5 knots with the engine at chugging at 2.4 k revs. With any headwind or in choppy seas, I'm lucky if I make 3 knots. In this part of the world where tidal currents across channels and harbour entrances regularly clock 6 knots and more, that feels inadequate.

I was wondering what was the choice of engine size for other owners of the same model, and the results they got.

Many thanks for all input.
Rafa
Rafael
s/v Nausikaa
SSR 30570, sail GBR 4619L
F30 CK (Hoyt), wishbones, centreboard, G10 rig. Built by Fairways Marine, Humble, UK, '82
Beta 16 hp with two-blade prop
User avatar
seadago
 
Posts: 79
Joined: Sun Jun 03, 2012 8:42 am
Location: Lowestoft, England

Re: engine sizing for an F28 ck Hoyt

Postby BillSmith » Wed Sep 25, 2019 10:41 pm

Is your boat bottom clean?
Bill & Carole Smith
Freedom 35 Pedrick
SweeTART
Coyote Point
San Mateo, CA
BillSmith
 
Posts: 61
Joined: Tue May 24, 2016 7:56 am
Location: San Mateo, CA

Re: engine sizing for an F28 ck Hoyt

Postby seadago » Thu Sep 26, 2019 4:35 am

BillSmith wrote:Is your boat bottom clean?

Hi Bill, as a whistle! Copper-coated.
Rafael
s/v Nausikaa
SSR 30570, sail GBR 4619L
F30 CK (Hoyt), wishbones, centreboard, G10 rig. Built by Fairways Marine, Humble, UK, '82
Beta 16 hp with two-blade prop
User avatar
seadago
 
Posts: 79
Joined: Sun Jun 03, 2012 8:42 am
Location: Lowestoft, England

Re: engine sizing for an F28 ck Hoyt

Postby andygc » Sun Sep 29, 2019 2:56 am

First, where did you get 3 tonnes from? Feronia weighs just over 5 tonnes on the crane with a full fuel tank, empty water tank, catering equipment, rubber dinghy, warps, fenders, a couple of bottles of wine, part bottles of gin and whisky, a few beers, bedding and some clothing. The engine is a Beta 20 fitted with a Featherstream prop. The prop is an inch smaller than Darglow advised, but it came off my last boat and was repitched to suit. I can't quite wind it up to the full 3,600 rpm, but top speed is 6.5kts in flat conditions. At 2,200 I get a little over 4kts. If you were running at 2,400 you were doing quite well to make 4.5kts - that will be the benefit of a clean bottom. If you can't go past 2,400 your prop is way out.
Feronia
(UK Freedom 30 cat ketch, build number 107)
River Exe, Devon
andygc
 
Posts: 77
Joined: Sun Jul 31, 2016 4:33 pm

Re: engine sizing for an F28 ck Hoyt

Postby seadago » Sun Sep 29, 2019 7:15 am

andygc wrote:First, where did you get 3 tonnes from? Feronia weighs just over 5 tonnes on the crane with a full fuel tank, empty water tank, catering equipment, rubber dinghy, warps, fenders, a couple of bottles of wine, part bottles of gin and whisky, a few beers, bedding and some clothing. The engine is a Beta 20 fitted with a Featherstream prop. The prop is an inch smaller than Darglow advised, but it came off my last boat and was repitched to suit. I can't quite wind it up to the full 3,600 rpm, but top speed is 6.5kts in flat conditions. At 2,200 I get a little over 4kts. If you were running at 2,400 you were doing quite well to make 4.5kts - that will be the benefit of a clean bottom. If you can't go past 2,400 your prop is way out.


Thanks for your comments Andy. Confused though. 3 tonnes is the displacement of an F30 according to the literature; 7000 lb. Just over. At least the ones built by Tillotson-Pearson in the US. As far as I know (and I may be very mistaken!) engines and props are sized, in among other factors, according to displacement tonnage, not mass of the vessel. When I selected the engine, I added 30% displacement load factor to the nominal 3.1 tonnes, expecting it would put me more or less in the right ball park figure re engine size, following the 4 HP/tonne rule.

But you are right, the prop seems to be right out. This is very annoying as the prop specs were recommended by the manufacturer for my engine/gearbox combination. I have a two-blade 14 X 9 RH. On speaking with another supplier about this problem, I was informed that prop was inadequate, and the cause of not being able to rev the engine through its full RPM range; it starts cavitating beyond 2.5K RPM. They recommended instead a 3-blade 12 X 8.5! This supplier assured me with that 3-blade prop i could get 5.3 knots @ 3.500 RPM.

Evidently, switching from a 2 to a 3-blade prop will have implications whilst under sail, but if I have to compromise, I rather lose half a knot under sail and gain a knot on the engine.
Rafael
s/v Nausikaa
SSR 30570, sail GBR 4619L
F30 CK (Hoyt), wishbones, centreboard, G10 rig. Built by Fairways Marine, Humble, UK, '82
Beta 16 hp with two-blade prop
User avatar
seadago
 
Posts: 79
Joined: Sun Jun 03, 2012 8:42 am
Location: Lowestoft, England

Re: engine sizing for an F28 ck Hoyt

Postby Castaway » Sun Sep 29, 2019 8:00 am

You might consider a KiwiProp. It has three blades, with adjustable pitch, and which feather. Not quite so efficient as a fixed three blade one, but as cheap and also repairable. The feathering means no discernible loss of speed when sailing. We are very pleased with ours, which we have used for four seasons now. It needs greasing once a year, but otherwise low maintenance.

Regards,

Gerald

PS, Castaway has a nominal displacement of 5.5 tons, but is about 7.5 tons in cruising trim; this is the displacement that has to be moved through the water. Under power, we can get 6.5 knots max, and more revs just sinks the stern further. Sailing, we can get 8 knots in good conditions!
Gerald Freshwater,
s/y 'Castaway', (UK F35 cat ketch, centreboard, 1987)
Lerwick Boating Club
Shetland Isles, Scotland
User avatar
Castaway
 
Posts: 205
Joined: Sat Dec 13, 2008 8:40 pm
Location: Lerwick, Shetland Isles

Re: engine sizing for an F28 ck Hoyt

Postby seadago » Sun Sep 29, 2019 8:26 am

Castaway wrote:You might consider a KiwiProp.
[...]
Regards,

Gerald

PS, Castaway has a nominal displacement of 5.5 tons, but is about 7.5 tons in cruising trim; this is the displacement that has to be moved through the water. Under power, we can get 6.5 knots max, and more revs just sinks the stern further. Sailing, we can get 8 knots in good conditions!


Thanks Gerald. I did at the beginning. Although Beta published guidelines recommended (at the time) a Kiwiprop as an option, I got in touch with Kiwi in the UK and they said it was not appropriate for my engine/gearbox combo; i.e. a Beta 16 HP with a TMC gearbox @ 2.1:1 ratio.

It's a bit of a no-win situation with any variable pitch prop. I don't have enough clearance between the end of the prop shaft and the rudder. And any folding prop is out of the question. Even if a Kiwi was appropriate, the length of pop hub would force me to cut a notch out of the leading edge of the rudder to fit. Same with a Darglow Featherstream. Maybe that's what I'll end up doing!
Rafael
s/v Nausikaa
SSR 30570, sail GBR 4619L
F30 CK (Hoyt), wishbones, centreboard, G10 rig. Built by Fairways Marine, Humble, UK, '82
Beta 16 hp with two-blade prop
User avatar
seadago
 
Posts: 79
Joined: Sun Jun 03, 2012 8:42 am
Location: Lowestoft, England

Re: engine sizing for an F28 ck Hoyt

Postby andygc » Wed Oct 02, 2019 7:28 am

I was puzzled by the weight in the literature. Hard to sea how a boat with 1.5 tons of lead ballast and the high volume hull that the 28 CK has can weigh in at 3 tons.

The FeatherStream should fit without needing any modification. There's two versions of the anode, pointed and flat. The flat one fits, but not the pointed one. Darglow would be able to recommend a prop for the engine you have, although they might observe that it's a bit underpowered.
Feronia
(UK Freedom 30 cat ketch, build number 107)
River Exe, Devon
andygc
 
Posts: 77
Joined: Sun Jul 31, 2016 4:33 pm

Re: engine sizing for an F28 ck Hoyt

Postby seadago » Wed Oct 02, 2019 8:23 am

andygc wrote:I was puzzled by the weight in the literature. Hard to sea how a boat with 1.5 tons of lead ballast and the high volume hull that the 28 CK has can weigh in at 3 tons.

The FeatherStream should fit without needing any modification. There's two versions of the anode, pointed and flat. The flat one fits, but not the pointed one. Darglow would be able to recommend a prop for the engine you have, although they might observe that it's a bit underpowered.


Thanks Andy.

I recall including the Darglow as an option when I was researching prop/engine combinations, and the issue with the length of the prop hub, taking into account not only the distance between the hub and the rudder, but also the minimum clearance required between the prop an the keel. Will look at it again.

And yes, a Beta 16 is too small. Tough choice. Upgrading to a 20 HP (GBP 4.5 k new) plus a variable-pitch prop (another coupe of grand), plus fitting etc, will set me back 8-9 grand. At the moment, too much for my pocket, to spend in that little boat in exchange for a couple of knots.

Will start changing the prop, and measure relative improvement.

Thanks again
Rafa
Rafael
s/v Nausikaa
SSR 30570, sail GBR 4619L
F30 CK (Hoyt), wishbones, centreboard, G10 rig. Built by Fairways Marine, Humble, UK, '82
Beta 16 hp with two-blade prop
User avatar
seadago
 
Posts: 79
Joined: Sun Jun 03, 2012 8:42 am
Location: Lowestoft, England

Re: engine sizing for an F28 ck Hoyt

Postby andygc » Mon Oct 07, 2019 4:43 pm

Rafael
Here's my FeatherStream with the flat anode. No modification to the rudder needed.

Image00001.jpg
Image00001.jpg (238 KiB) Viewed 124 times


Regards
Andy
Feronia
(UK Freedom 30 cat ketch, build number 107)
River Exe, Devon
andygc
 
Posts: 77
Joined: Sun Jul 31, 2016 4:33 pm

Next

Return to Engines and Mechanical Systems

Who is online

Users browsing this forum: No registered users and 4 guests

cron