prop pocket/tunnel problem solutions

I just took over management of a 30 Luhrs express open. After extensive repairs & service in the yard, she performs miserably!

The back story: Boat hauled with complete running gear service: pulled straightened shafts, reconditioned props to Class S 29.5' Pitch, couplings faced & lapped, new cutlass bearings, straightened rudders, lapped props onto shaft, engine/shaft alignment @ .002" at both couplings and new Tides drip-less seals units. Engines just finished full service, they run great. Full bottom job. So the machinery is 100% good and within spec.

She has one of the worst mid-range performance characteristic of any boat I've run in 30 years. I've run plenty of tunneled and pocketed boats. They all have their flat spots and idiosyncrasies. Most issues are tolerable or manageable in nature. This one is different.

Specs:
LOA 31' 6" 34 w/pulpit
Length on waterline 27'7"
Draft 2'3"
Dry weight 14,000
Tested weight 16,000 (fuel & crew. The boat was stripped clean empty, not poorly ballasted or excess weight aft)
Yanmar 6 cyl turbo 188hp @ 3200 232 Hp @3800
Props 19" x 29.5" pitch 4 blade zero cup
Gear 2.48:1
Prop pocket aperture 20-1/4" at ransom
Aperture at props: 1" to 1-1/4" tip clearance prop tip and hull inside tunnel props appear slightly eccentric to pocket axis but running gear is properly targeted and aligned.

RPM
1000 @ 5.5 kts
1200 @ 6.0 kts
1600 @ 6.0 kts
2000 @ 8.2 kts
2600 @ 10.0 kts
3000 @ 14.6 kts
3200 @ 21.7 Kts
3800 @ 27.5 kts

Basically at 1100-3100rpm she goes bow straight up; wallows; and digs a giant hole. She barely responds to trim tab input until 3200rpm. At 1400 a rapidly growing wake that becomes expetionally disproportional. Bow is at a far too high attitude.

At 3200-3800rpm she runs great, slightly bow up, overall she comes to life. Runs cruise and WOT at spec. The low & midrange is horrible.

Has anybody had similar problems and arrived at a working solution?

Here's some of my potential solutions.

1) Change wheels: I'm thinking smaller diameter props. I found a set of 5 blade Nibral 18"D x 27"P heavy cupped wheels...increase tip clearance and water flow.

2) Add a tunnel extension to transom

3) Add keel extension to hull between prop pockets to get a little more running surface and possibly some stern lift

4) Hull wedges at transom. There's a little small wedge about 1" by trim tabs. Put on a full wedge.

5) Extend trim tabs length. They are already large for this vessel. They are flush with transom at this time. Maybe extend them 8" or so.

Does anybody have experience with tunnel hull boats squatting so severely and performing poorly low & mid range.

 

Well, you have to compare apples to apples, it doesn't matter what the other hulls you've experienced run like, just what others of this breed do. It might useful to get a line on that from speaking to owners, assuming the tunnel set-up is standard. The boat may be excessively heavy, especially if it is stern heavy would be detrimental, nothing that depends on dynamic lift is going to have any appreciable effect at those lowish speeds where the nose points skyward, extra bouyancy aft could help that. Or shifting weight forward, or better still, out of the boat completely.

 

Some photos of the boat & tunnels would really help

 

The slenderness ratio (LWL/Displacement^0,33) is very low, ie the hull is far too heavy for its length; this is the main issue here. Bad design of propeller tunnels may aggravate the hump problem, so in addition to the pics requested by JSL, we need more detail info on the shape of the tunnels, and the position of the props.

 

Without getting into everything else, your power to weight ratio the lower speeds is miserable. Way too heavy for an empty boat. Your boat weight for the setup should be closer to 12k. So her jump from hull speed to planning requires wot. I would look for ways to clean hull and make it lighter. Apart from that trim tabs. Very difficult to put bigger props and engine.

 

Move weight forward, batteries etc...

 

Beam, dead rise and bow up degrees at varying speed would help

 

The figures quoted for prop pitch, rpm, and gearbox ratio look wrong, it equates to 26% prop slip at full throttle, and a whopping 50% at 3000 rpm.

 

Yes, those props are heavily loaded; it's obvious when checking cavitation criteria as well. So reducing diameter is NOT an option. Let's see some pics and/or drawings of the bottom and tunnels. And get rid of all the unnecessary stuff onboard, or lengthen the bottom.

Problems with propeller pockets/tunnels pop up rather frequently here, particularly when applied to short, heavy hulls. It seems to me that the designers have got the inflow Picture wrong in most cases. At hump speed, where the propeller loading often is the highest, the propeller disc acts as a fluid sink, and the streamlines approach the disc in a strong Three-dimensional flow.

I attached a simulation picture from a case with slightly higher propeller loading (roughly 150 hp at a hump speed of 11 knots and a prop dia 16"). Here you can see that the inflow streamlines close to the hull surface are traversing the tunnel/bottom edge at high angles. This leads to strong disturbance in terms of whirling losses and loss of static pressure. This type of propeller tunnel is a disaster for the propeller performance, yet you find them all over.

The pic is just for illustration of the phenomenon; I used a very coarse mesh that is too rough to show details close to the hull surface. The lower pic is truncated at the fwd limit of the tunnel and shows two diametral streamlines sweeping into the propeller disc from the sides. This is where there should be a generous radius between bottom and tunnel!

 

Thank you all for your contributions.

When the boat was sea trialed it was stripped clean of everything but USCG required safety equipment, myself, mechanic, handheld GPS and a cup of coffee. So she was as light as she will ever get.

The RPMs and other data are dead on measured.

I did forget beam, but everybody does seem to have figured it out...pudgy little hull. BEAM 10'10"

I do not have any pictures of the hull below the waterline.

We are looking at relocating batteries as far forward as possible.

I did find a set of CY5 propellers 18" Dia x 26" pitch 5 blade the EAR is .826 a bit better than the the current 4 blades with estimated EAR of low .700 ish

This is what Im thinking about the CY5 props

- Almost doubles tip clearance
- More blade area and better loading
- Better waterflow through tunnel (I know slight)
- Less pitch allows engines to spin up quicker
- Not really worried about WOT speed just improving mid-range and cruise
- Once boat is loaded with owners stuff will require less pitch due to increased weight.
- Prop shop can play with pitch and cup to dial in prop

Again thanks for the input...

 

I have this same boat with same power, and speed problems. (Luhrs 30 - 2 x Yanmars) Good description of specs and speeds above. Looking to make hull mods to make boat faster. Need some thoughts - considering:
- Adding a lengthy, down dipped , permanent trim tab in between the pockets (see YouTube) "Luhrs 30 Kahuna", at end of video.
- Narrowing the upper pocket outlet at the transom, ie, taper the outlet, like a fighter jet exhaust / thrust nozzle.
- Flatten and taper downwards the upper section of the tunnel outlet to minimize the prop wash into the air.
- Installing a "fence" just in front of the outboard hull tunnel entrance, to channel any transvers side flow of water into the tunnel (like a chine). Same device used on some aircraft wings to keep the air from running sideways off the wing tips.
Lots of photos embedded in attached Word document.
Thanks, Mike