Water spray from dinghy

[Red Raven]

A question for those who have an R27 with the Ranger RIB260 on Weaver davits.

Does your dinghy get hit by water while under way at high cruise? If so, do you have any suggestions to manage this?

We get a bit of spray into the cockpit such that the rear seat is unusable while underway without getting wet. This only occurs on the port side where the dinghy inflatable tube extends past the dinghy transom. The seawater coming around the stern of the tug hits the dinghy because the tug stern sits "squats" below the surrounding water at planing speeds. This occurs even after trimming the bow down to maximum. Since we have the factory setup I was interested to find out other's experience with this who have this setup.

Everything is fine at low "displacement" speeds.

I'm considering some sort of shield for the spray but would also like to know if this is the normal experience for others.

Thanks,

Curt

 

[steven_ickman]

I have a Cutwater 28 and almost always end up with water in my dinghy as a result of spray while underway.

Steve

 

[Red Raven]

Thanks Steve. Yes, I get spray in the dinghy as well but don't find that much of a problem. Just wash it down when we get back. Some spray however is getting in the cockpit on the rear bench such that a guest cannot sit there while under way.

I'm not sure how similar the swimstep/transom/cockpit arrangement is on the C28 to the R27. Do you get any spray in the cockpit?

Curt

 

[steven_ickman]

We do get a little spray in the cockpit while underway from about 12-13 knots on. I've only had one person move into the cabin because they said the spray was too bad. We only have 60 hours on our boat though so others might be able to comment on how common an issue this is.

Steve

 

[Red Raven]

Thanks. This sounds similar to our experience. Most of the cockpit is fine, just the seat at the stern gets wet. People just move forward by the door.

Thanks for your response.

 

[steven_ickman]

Yes.. It's mainly the transom seat that gets wet.

 

[dialtone]

This occurs on our cw30. I noticed that keeping the dinghy properly inflated helps (low air pressure drops the dinghy lower to the water line).

Perhaps the factory can recommend some solutions? I'm guessing there's a fine balance between having the dinghy mounted higher (will be harder to remove from davits since there is less buoyancy) versus having it "just right".

While underway I also pay attention to trimming the vessel so that one side isn't dragging the dinghy. With a little patience we get it under control.

 

[Boont Boater]

Definitely check the pressure. That can make a big difference.

Can someone tell me the correct pressure for our Ranger RIB's? For some reason I never quite got that.

Jeff

 

[Red Raven]

The correct pressure is printed next to the valve. The dinghy definitely rides on the swim step better when the pressure is correct. Less bouncing as well as riding higher. Any more pressure though and I will be above the max. Trimming to keep the port side up helps a lot as well though It still hits and results in spray.

I thought about raising the dinghy with higher davits as well but agree it would compromise the ease of attaching and removing the dinghy. I also thought about moving the dinghy a few inches to starboard. I haven't checked yet but I would be concerned about the dinghy bow extending to far starboard and potentially interfering with rear visibility, dock pilings, or while on the highway. This still may be an option though.

Shorter term I'm thinking of some sort of spray shield I can temporarily attach.

Any other ideas?

Curt

 

[pleeson]

This is IMO the result of the hole our fixed (downward angled) shaft inboards, with large props, dig for themselves and can't climb out of. I doubt larger trim tabs would assist unless perhaps they were longer rather than wider - you can't get too close to the prop or the edges of the hull without screwing up the hydraulic flows. However, I wish we could more easily try ridding the swim platform of the rear lower lip that does drag in the wake and cause most of the curl creating this problem. It seems to me a solid bottom swim platform, thicker at the hull and tapering to the bottom of this existing lip, would actually act as a bit of a planing surface to help support the boat at speed and reduce the squat we experience, the spray created by the rear lip, and it's related drag. Many newer outboard boats now have this hull "extension", in this case it would be stepped to avoid drastic impacts on prop flows and drag. BTW - I know from experience with the good folks at Port Boathouse that you can add at least four inches of height to the lower davits and still have no issues taking the dinghy on and off the clips while its in the water. This helps markedly in the spray collection and deflection problem but is not a total solution - that lower lip of the swim platform really throws the water around at some higher speeds - irrespective of trim tab positioning. I think if I ever had to do a long high speed crossing I'd pull the damn dinghy into the cockpit - frustrating because I think a little more engineering and computer modelling could resolve this.

 

[Red Raven]

Hmm..Interesting analysis about the swim step as the cause. I wonder if a beveled "filler" block could be attached under the rear edge of the swimstep to smooth the flow?

 

[CORPSVET]

I hate to fuel the complaint department, but I have been wondering the same as "pleason"about modificationso to the swim platform. Our boats are so great, but both Cuts and Tugs suffer with dingy drag. I have brought the dingy up on the back of the boat - not fun, but made a big difference - on days I knew I would be spending time on step.
I know the factory is aware of this problem, a fix takes time. Maybe an easier way to lift the dingy up out of the water is an answer.
Smooth sailing
Dave

 

[colinm]

What about towing the dinghy at speed? I have tried towing the dinghy because of the spray problems although I was more worried about the pressure on the dinghy more than things getting wet. It seemed like towing the dinghy didn't have an effect of fuel efficiency at 12 knots and I had it positioned just behind the "wave."

Colin

 

[CORPSVET]

never towed a dingy before, can't figure out it keeps from running into the boat when you slow down! 😛 😀
wonder also how it would ridw at 20 to 24 knots?

 

[steven_ickman]

The question for me about towing a dinghy is does towing increase the fuel efficiency over using a davit system? One obvious advantage to using a davit system is you can cruise at higher speeds which for many is enough to make the davit a better option. Comparing the two approaches the weight of the dinghy is the same either way so it's really water resistance versus air resistance. The weaver system in particular sucks because it essentially turns your dinghy system into a sail and adds drag. Question is does the drag of it being on the davit cost more than the drag from pulling it through the water?

Steve

 

[Red Raven]

Yesterday I added additional air pressure to the dinghy a little above what is stated on the valve and that resulted in some improvement on the spray and bouncing around. It still hits the water with lots of spray when the tug hits waves at speed but rides pretty well on smooth water. When inflated at the recommended 3.6 psi it is pretty soft. 4 psi works much better but I don't know if I am damaging the dinghy at this pressure. It still seems OK and is not real hard. There is still considerable spray from the swimstep and swim rail even when the dinghy is not hitting the water. This is all at 16 knots.

I also found that the dinghy is mounted about 2 inches port from center. I am not sure why this is but moving it two inches to starboard would help stop the spray. Since the tug lists slightly to starboard from the weight of the dinghy motor on the mount moving the dinghy to starboard would help that as well as the spray.

The wind drag is a bummer but I think the cockpit canopy helps.

Curt

 

[Red Raven]

Problem solved!

I added a strap to the dinghy port side transom lifting eye and one to the dinghy interior lifting bow eye. Each strap then runs diagonally to the base of the stern rail of the tug. These straps are then pulled really really tight actually lifting the dinghy up almost two inches.

It turns out the Weaver yokes have fair amount of play due to mounting on the air filled tubes. Even when the tubes are overinflated a bit the weight of the dinghy causes the yoke to flex the tube resulting in a low sitting very bouncy connection. Previous to my first post on this I had tried straps connected to the tube tie down rings. These rings cannot take the required tension to lift and hold the dinghy and since they are mounted to the soft tubes there was still a lot of movement of the dinghy back forth in the waves. In rough seas at speeds above 12 knots the rear of the port dinghy tube would hit the water and violently rock back and forth with greater and greater force as it hit each wave. Something was going to give.

With the new setup the dinghy sits solid as a rock with no movement at any speed and never hits the water. The key is the straps have to be tight enough to lift the dinghy such that the the dinghy is not resting downward on the yokes (it is actually pulling up on the yokes). To do this the straps must be connected to the rigid hull of the dinghy and the hull of the tug.

Of course we still have the problem with the spray from the lip of the swim step. That seems to occur at speeds about 14 knots. I'll have to see if anything can be done about that after we take the boat from the water next month.

Curt

 

[knotflying]

Are you trimming the boat enough to lower the bow, hence raising the stern a bit? This keeps the swim platform a little higher above the water.

 

[pleeson]

You can trim to the cows come home, move weight forward, and play with your speed, but when on the "step" this hull drops the back of the swim platform into the water flow and creates these issues. I sure would like to see a solid tapered bottom sans the current lower rear lip on the swim platform and maybe some experimentation with extended trim tabs to see if the bow can be brought down a little more. In any case, these extra straps seem workable for longer high speed crossings, and my roughly 3 inches of additional height added to the Center section of the davits seem to make things bearable in the dinghy water scooping and spraying department... Definitely a weak point on overall design - nice to be able to go fast when it's called for, it just raises some other issues with the current design and layout, especially with RIBS on a Weaver mount.

 

[Red Raven]

I almost always trim bow down as much as she will go at any speed above 7 knots or so except in cases running downwind in rough seas etc. when you need to keep the bow up a bit for handling.

At this point, everything rides smooth with no spray into the cockpit up to 14 knots or so at max (bow down) trim. For us this is about 3400 RPM. Interestingly the bow raises a bit going from 14 to 16 knots and that is when the swim step drags and the cockpit spray forces passengers inside.

Andrew has stated in previous posts that more trim could make the boat unstable at these speeds. So I don't think more trim is the answer. It is my understanding the bow needs to be out of the water at these speeds to handle properly. (Which in my opinion it does extremely well!). Some sort of fairing under the swimstep might help but I'm pretty happy with cruising at 14 knots as our high speed cruise.

On a side note, we hit 22 knots the other day at WOT with 4 people, all their gear, a dirty bottom, and 70 gal of fuel in the boat! It is amazing what a helpful current and a tail wind can do! 😛

Curt