Engine height dilemma

[sydneyribber]

Hi

Apologies for another one of these topics. But I can’t seem to find another post based on my dilemma.

So I have a Suzuki DF175 on the back. Currently in centre holes so have the option to move 2 up or 2 down.

I will have to go out and grab a photo but the cav plate looks to be pretty flush with the bottom of the hull.

My issue is as follows. When I trim out fully to what to me seems right for ideal ride, bow lift etc I end up with a bit of cavitation. This to me says try lowering the motor so when I get optimal trim the prop is lower in the water.

However when I hang off the back to have a look while under way the plate is actually under the surface and not riding flush. So my concern is if I lower it I will be adding considerably more drag etc.

Could prop selection also be an issue?

Prop is a Suzuki stainless 3x16x20L

[Xk59D]

What revs are you hitting flat out and with typical load?

Suzuki seem to like running deep, maybe due to the large props they swing.

I raised my engine up 1 hole to stop a lean to port but I introduced caviation if trimmed right up in tight turns as a by product but I just live with it as mines never cavitates unless as per above...I.e hardly ever does it.

Btw suzukis are a pita to move up or down as the engine needs to completely come off, there is no slide holes like most engines seem to have..or at least my df200 doesn't anyway.

[sydneyribber]

Originally Posted by Xk59D

What revs are you hitting flat out and with typical load?

Suzuki seem to like running deep, maybe due to the large props they swing.

I raised my engine up 1 hole to stop a lean to port but I introduced caviation if trimmed right up in tight turns as a by product but I just live with it as mines never cavitates unless as per above...I.e hardly ever does it.

Again I’d need to get it on the water to check the revs.

At the minute mine is starting to cavitation when running straight.

I’m wondering would a foil help with the cavitation?

[Xk59D]

Prop might be the issue but that could get expensive quickly to prove it. Need to know your revs to comment further really and a piccy.

I have the same prop on my df200 and it is more or less bang on the 6k max rpm on my boat when trimmed out now.

I thought I had a picture of my cav plate but I can't see it in my Google pics, I'm sure my cav plate is level now or maybe 1/8 above but that doesn't mean your boat will like it.

Perhaps you are simply trimming out too much, you should trim out until the speed no longer changes. Once it stops raising I back it off a touch and that is pretty much bang on for fuel economy and ride in my own boat. I don't have a planning pad though so ymmv

Edit....here ye go, I went 1 hole up from here

http://www.rib.net/forum/f36/to-rais...tml#post666630

[sydneyribber]

I’ll get it out today or tomorrow and get a few pics / readings. I’ve been in ribs of various sizes and designs most of my life with the family so I sort of just know where my ideal trim is and how it should ride.

At the minute I’m aiming for enough trim so that the steering is nice and light and provides enough bow ride for a comfortable ride.

However as this is my first aluminium hull it is considerably lighter than the red bays I’m used to so maybe I need to take a new approach. If I keep the trim any lower then the bow is just too heavy when hitting the ferry wakes here in Sydney Harbour which makes me really back off and take it easy. Which I never had to do in the past.

The boat is a high field 6.4m, boat and engine comes in at around 750kg which is pretty light. And normally there is little weight in the boat and just myself on most outings.

The engine came off an ex Marine rescue Sydney boat which is why I questioned the prop, may have been perfect for the heavier boat running twins but maybe not ideal for my setup.

Anyway I will get a few readings and report back

[sydneyribber]

I should mention that with 3-4 people in the back of the boat the ride is much more like I’m used to and I have slightly least cavitation.

[sydneyribber]

Sorry slightly less. Autocorrect is my enemy today.

[Xk59D]

Well our 2 boats couldn't be any different then, I'm 1650kg plus trailer.

Anyways, check out my thread above mean time if it helps.

Btw...is it a stand rotation engine? If you got the wrong one and the engine is offset the "right" way that might be your issue. Some Suzukis are programed so you can change it if need be with a prop change and a visit to a dealer, i don't know what engines they do this with though so you will need to look into that if this is the case.

Without seeing pics or knowing rpm, as your boat is light coupled with a big prop then may need to go in a hole. I.e there is too much stern lift with the 16" prop.

[sydneyribber]

I’ll have a look at the thread and report back with some findings.

It’s a counter rotating engine but I didn’t think that would affect too much as it’s a single engine setup. At the minute I’m pretty sure it’s rigged with no offset. Looks dead centre on the transom.

[Xk59D]

I don't know if that is right or wrong on the centre mounting, highfield probably best to ask. Many schools of thought on that one and I'm no expert on it. I only know ribcraft and my last avon were offset. Wouldn't think being centre mounted would cause cavitation though.

Anyways, need pics and rpm before doing anything else I would suggest.

[beerbelly]

my osprey had a counter rotating yam 130 and was offset to port but I thought this was to keep it from pulling to one side when trying to go straight. probably not related but my warrior fishing boat had a Suzuki 80 and would cavitate in a straight line when trimmed all the way down .turned out it wasn't all the way down there was a plastic cam type thing on the tilt tube that worked a switch to stop the pump at full tilt and it had turned slightly cutting the pump early repositioned and a cable tie got my motor trimming down a couple more inches cured it .I also read on the internet that trim perimeters can be set electronically via the control box

[phoenixpete]

Not sure a left handed prop is right on a single engine installation .3x16x20 seems a bit big,my df175 on a 7mt Humber has a 3x14/3/4x23 stainless that is spot on.I think you can do something to the electrics to alter to a right handed rotation then get a right handed prop?

[sydneyribber]

Im sure someone will correct me if I'm wrong, but I really don't think the rotation matters on a single install.

I definitely think my issue is one of or a combination of:

1. Engine Height
2. Weight
3. Prop selection

I have very little if any issues with it not running straight or leaning to one side which is where I would expect the offset to help. the issue really comes down to achieving correct trim causes the prop to cavitate when running a straight line.

Any way thanks for everyones help so far. going to hitch it on and grab a few pics and readings today so will report back shortly.

[Xk59D]

If the engine is in the centre it won't matter, if it is offset the wrong way it might cause something weird but not convinced it would cause only cavitation.

[sydneyribber]

Ok

Apologies I did not have a stick or anything to use so see what you can make from the photos. also I have attached a short youtube link to the engine underway.

when trimmed up and running WOT or close to I was definitely hitting 6000 it was a little choppy so I actually think it might go over to maybe 6100

I really only ever cruise along at 25 Knts or so, vary rarely if ever do I run flat out, possibly a little bit of chine walking.

https://youtu.be/xjgHfIIxoBU

[beerbelly]

could go down a hole are you sure its trimming all the way down ie under the saddle is the outboard hitting the cast in stop on the saddle bracket could be camera angle but looks like it could go a little further trust me it doesn't take much mine went an extra inch made a massive difference

[sydneyribber]

Yeah the engine definitely trims all the way down. and I normally take off trimmed fully down and then trim up until desired. unfortunately my idea of desired is cavitating which is why I think I need just a bit more trim.

I actually trimmed it up slightly in the pics to try and have it level with the bottom of the hull. otherwise it has a bit of a negative angle if that makes sense.

so my thoughts are to drop it down. but then when you watch the video you can see that it already looks too low?

[MatFromPoland]

Highfield has flat pad on the end.

It likes more engine lower a bit.

Mine is max down.
It is my personal observation - I'm not expert or phd at naval arch.

But did tests with lower and upper mine engine.
Anticaviataion engine plate is at pad level I suppose (in my Higfield)

But mine is Etec and smaller boat.

>>>>>>>>>>>>>>>>>

It is picture of the winterize procedure - this is the only way to have engine down (trailer is on support - without them I can't lower engine full down)

[sydneyribber]

Hey mat. Good to hear some advice from another high field owner.

My gut tells me drop it and give it a test. But at the same time I hate to admit it or even think about the cost but I’m pretty sure I need to look at the prop also.

Can I ask what size you have and what length on the engine shaft.

[beamishken]

You can get specific props that help induce bow lift it may be worth looking at one of those. Ive no specific experience but I have seen them for sale